Rebrand: README and documentation #1029

Part of #1024. Step 5, after the images.

Apply the naming rule: full name in titles and first mentions, then "the extension".

• README.md: H1, the open-source intro, hero alt text, theme and feedback paragraphs.
• docs/index.md, docs/release-notes.md, docs/user-manual.md, docs/blog-posts/index.md.
• User manual pages (about 20 pages, 1 to 4 mentions each).
• Docs that tell users to open the "DocumentDB for VS Code" output channel: rename to "DocumentDB" (matches the code change).
• 1.0 release notes and blog post text; sync the repo blog copy with the documentdb.io version (rebrand and telemetry sentence).
• Package READMEs (packages/vscode-ext-webview, packages/vscode-ext-webview-fluentui). They reach npm only with the next package publish.
• Leave historical content unchanged: 0.x release notes, CHANGELOG.md, docs/ai-and-plans/**.