[kattni]

Migrates the Briefcase documentation build from MkDocs to Zensical, using native Zensical configuration.

This depends on beeware/beeware-docs-tools#288, which moves docs-tools to Zensical. Until that PR merges, the docs dependency group is temporarily pinned to its branch (kattni/beeware-docs-tools@tooling/zensical-migration) so this PR can build. The pin will be reverted to docs-tools main once #288 is merged; this PR shouldn't merge before then.

Changes

• Native Zensical config. docs/config.yml and docs/mkdocs.en.yml are replaced by docs/zensical.toml (shared) and docs/zensical.en.toml (English), generated with the docs-tools convert_mkdocs_config converter. The literate-nav docs/en/SUMMARY.md is replaced by a native nav in zensical.toml; its one glob (how-to/publishing/*) is expanded.
• Host Platform Support table headers. The 12 platform reference pages linked the table header with a hand-written relative href in raw HTML. Zensical rewrites raw-HTML links relative to the source file, so those links came out one level too high (a 404 on Read the Docs). They're now Markdown links via md_in_html, which render the same and are validated by the build. md_in_html is also added to the pyspelling Markdown pipeline so the link targets aren't spell-checked.
• Housekeeping. .gitignore ignores the generated docs/_build.*.toml; tox.ini drops the MkDocs 2 warning environment variables; .specify/memory/constitution.md now says Zensical instead of MkDocs.

Verification

• tox -e docs-en (strict) and tox -e docs-lint pass against the pinned docs-tools branch.
• Compared with a MkDocs build of main: page text, resolved link targets, page titles and the sidebar nav match on every page. The only text differences are 4 contribution-guide pages, where docs-tools' shared content now says to edit nav in zensical.toml instead of SUMMARY.md. Remaining differences are theme-level (for example, Zensical's compact search box).
• An English plus synthetic French build also passes, with translated shared content and all static files copied.

Notes for reviewers

• AGENTS.md still describes the docs as MkDocs ("Docs: MkDocs", "MkDocs documentation"). It's left unchanged here because AGENTS.md is only edited on explicit direction; it should be updated to Zensical, here or in a follow-up.
• pymdownx.magiclink.repo is "toga", carried over unchanged from the old config. That may be unintended for Briefcase.
• Zensical ignores some carried-over settings: autorefs resolve_closest, and the omitted_files and unrecognized_links validation keys. They're kept for parity with docs-tools' own config; autoref resolution matches the MkDocs build regardless.
• [tool.rumdl] flavor = "mkdocs" in pyproject.toml is left alone; it selects a Markdown dialect, not the build tool.

PR Checklist:

• I will abide by the BeeWare Code of Conduct
• I have read and have followed the CONTRIBUTING.md file
• This PR was generated or assisted using an AI tool

Assisted-by: Claude Code (Claude Opus 5.5)