feat(site): add branded navigation and theme controls

[thxCode]

What type of PR is this?

/kind enhancement
/kind documentation
/area integrations

What this PR does / why we need it:

• Align Ask Devin, GitHub, version, theme and search controls in a compact documentation header.
• Use the operator logo's brand blue with graphite text and neutral surfaces throughout the site, including the dark theme, tables and code blocks.
• Replace the native version select with a rounded menu driven by the existing published-version manifest. Preserve the article path when switching versions and retain the homepage fallback.
• Add persistent Light, Dark and System choices, with an initial theme script and system preference support. Keep selected rows readable on hover and focus.
• Add Ask Devin as a DeepWiki link that opens in a new tab while preserving the current article. Move the existing search form into the header.
• Keep mobile navigation and search within the viewport, with a compact two-row header and visible focus states.

Which issue(s) this PR links to:

NONE

Special notes for your reviewer:

• Publication changes are isolated in fix(ci): publish charts and documentation as one Pages snapshot #707; this PR contains only the five site template, CSS and JavaScript files and can be reviewed independently.
• make site, make lint docs, the production-path site build and JavaScript syntax check passed. No documentation tests were added.
• Qwen implemented the UI against a scoped handoff; lead review corrected the mobile selector specificity and completed the latest new-tab requirement.
• Real-browser checks covered the three theme choices in light/dark modes, selected and unselected hover contrast, Escape focus return, theme persistence, actual search results, the published version menu and RC2 article navigation.
• A production-prefix build at a 375px viewport kept the header, version panel, tables and code blocks within the page width. Desktop and dark theme screenshots were visually reviewed.
• Ask Devin uses target="_blank" and rel="noopener noreferrer"; clicking it retained the article URL.
• Local previews without a publishing root omit the version menu. Published pages load actual versions, with visible loading and unavailable states. Language controls appear only where translated pages exist.

Does this PR introduce a user-facing change?

Refresh the documentation navigation with GPUStack branding, persistent light and dark themes, a published-version menu, header search and an Ask Devin link to DeepWiki.

[gpustack-code-review]

✅ OpenCodeReview: Review complete: 0 finding(s) across 1 selected item(s).

Reviewed 66c05de..02660b7 only; earlier commits in this PR were reviewed in a previous run.

[thxCode]

Review follow-up in 28f0ab2ee6f8d3bd12b858cbd849b2cb13e2c8c4 (fixup of dce9173).

Read all paginated feedback: one review, one inline thread and one conversation summary containing four additional findings. All five findings are accounted for:

• Duplicate dark palettes: addressed in code.css and site.css. Light and dark colors are paired once using light-dark(); the existing color-scheme still selects the active palette. This uses the CSS color function available in current browsers (Baseline 2024).
• Theme menu remains open: addressed. Selecting Light, Dark or System now closes the panel and returns focus to the summary.
• Escape summary lookup: unchanged. Every picker is emitted by site-selectors.html with an explicit summary; the reported failure requires a hypothetical future invalid template. Browser checks confirmed the current Escape behavior.
• DeepWiki URL configuration: unchanged. This site belongs to this repository; fork/repository-relocation configuration is outside this change.
• GitHub URL configuration: unchanged for the same reason; existing page-source links already use the repository URL.

Validation: make generate succeeded in a temporary checkout with the required module-path suffix and produced no generated drift; make site, make lint docs, node --check and git diff --check passed. Browser comparisons confirmed all 71 token/container styles retain their original colors, and 27 page selectors retain their colors, borders and shadows across six combinations of theme choice and system preference. Selection dismissal, focus return and Escape were checked in the existing localhost:1313 preview.

No documentation tests were added. The fixup is left standing for squash merge. CI for the updated head will run after push.

[thxCode]

Second review follow-up in 66c05de (fixup of dce9173).

The latest run emitted two findings: one inline browser-compatibility finding and one summary-only .bg finding. Both are addressed:

• Browser compatibility: accepted. The light-dark() implementation described in my previous reply has been replaced. Hugo now expands a single dark palette definition for each stylesheet into the existing explicit-dark and system-preference rules at build time. The published CSS has no light-dark() dependency and no template syntax; color values are defined once in the source.
• Code background: .bg now uses --code-bg, matching the active site theme.

Validation: make site and make lint docs passed; make generate in a temporary checkout produced no generated drift. Browser comparisons against the original CSS confirmed all 71 token/container styles and 27 page selectors retain their original colors across the six combinations of Light/Dark/System choice and light/dark system preference. The .bg rule was checked in all six combinations and follows the code background. No documentation tests were added.

The existing localhost:1313 preview remains running. This is a separate fixup; the earlier fixup remains in history for squash merge. Updated-head CI will run after push.

[thxCode]

Third review follow-up in 02660b7 (fixup of dce9173).

Read all feedback again: four reviews, four inline comments including replies, and three conversation comments. The latest run emitted three summary-only findings:

• Generated-file comment: addressed. The header now identifies the generated light rules and the manually maintained Hugo dark palette, and warns that regenerating the file would remove that palette.
• Duplicated code background: addressed. The base .chroma rule uses --code-bg; the redundant dark background override is removed. Both .bg and .chroma use the site variable.
• Shared light/dark token map and indentation: kept as a possible later refactor. Separate light/dark values are intentional; adding a map layer or mirroring the light rules as another template string does not fix a current rendering defect. The shared dark definition already prevents the original duplication between explicit and system rules. Browser comparisons confirm current token coverage and matching palettes. CSS indentation has no effect on the generated output.

Validation: make generate produced no generated drift; make site, make lint docs, node --check and git diff --check passed. All 71 syntax styles match the original palette in six manual/System combinations, and the shared code background follows the selected theme. No documentation tests were added.

All ten findings from the three completed review runs now have an outcome in this conversation. The two actual inline threads remain resolved. This fixup stays with the earlier fixups for squash merge; localhost:1313 remains running.