Add rendered documentation site (ReadTheDocs or GitHub Pages)

[codeforester]

Problem

base-cli has extensive documentation in docs/:

• api-stability.md — compatibility contract and deprecation policy
• adopter-readiness.md — production adoption checklist
• json-contracts.md — versioned JSON output contract
• output-contracts.md — output rendering rules
• typer-adapter.md — Typer migration guide
• security-threat-model.md and security-review.md
• migrations.md — breaking change migration notes
• platform-support.md — OS and Python version matrix
• integrations.md — Rich and OpenTelemetry opt-in
• cache-ownership-and-layout.md — runtime layout
• consumer-profiles.md — profile authoring guide
• extensions.md — extension and plugin system
• performance.md, local-config.md, releasing.md

All of this exists only as Markdown files navigable through GitHub. For a PyPI package, potential adopters expect a rendered documentation site reachable from the PyPI page (project.urls → Documentation).

Without a rendered site:

• The PyPI page has no Documentation link (or it points to the GitHub repo, which isn't the same).
• Search engines don't index the content as a coherent documentation site.
• Users reading the README are linked to .md files via relative paths — these work on GitHub but not when the README is rendered on PyPI.
• The docs/api-stability.md URL pattern doesn't resolve to anything for users who install via pip.

Proposed fix

Set up a documentation site using MkDocs or Sphinx with ReadTheDocs (or GitHub Pages):

1. Add mkdocs.yml (or docs/conf.py for Sphinx) configuring the navigation from the existing docs/ files.
2. Add a docs URL to pyproject.toml:

   [project.urls]
   Documentation = "https://base-cli.readthedocs.io"

3. Add a .readthedocs.yaml configuration file.
4. Register the project on ReadTheDocs (or configure GitHub Pages in the repository settings).

The docs/releasing.md file already documents the release process — it should be included in the rendered site so it's discoverable without knowing the repo structure.

Acceptance criteria

• pip show base-cli includes a Documentation URL that resolves to a rendered site.
• The rendered site includes the API stability policy, adopter readiness guide, migration notes, and JSON contract documentation.
• README relative links (e.g., [docs/api-stability.md](docs/api-stability.md)) either resolve correctly in both GitHub and the docs site, or are updated to use absolute URLs.

[codeforester]

Correction from Codex review: The acceptance criterion in the issue body references pip show base-cli as the verification step. Codex correctly points out this is the wrong signal — pip show reads installed metadata, not the PyPI project page. The correct check is: confirm a Documentation URL appears in the wheel's METADATA file (visible via python -m pip show base-cli only after the URL is added to [project.urls] in pyproject.toml) AND that the URL resolves to a rendered site on PyPI's project page. The acceptance criterion should be updated accordingly before implementation.

[codeforester]

Implementing Step 5 with MkDocs Material and GitHub Pages:

• Add a canonical https://basefoundry.github.io/base-cli/ site with navigation over all existing docs.
• Add strict MkDocs builds and repository-link checks to CI.
• Publish the Pages artifact from main with the protected github-pages environment.
• Add the Documentation project URL to package metadata and update README/security links for PyPI-safe navigation.
• Verify wheel metadata contains the canonical Documentation URL.