docs: publish the documentation with Zensical on Read the Docs

[allen0099]

Part of #81. It does not close the issue, because the site only goes live once the Read the Docs project is imported and builds master.

What

• zensical.toml builds a site from the existing guides:
  • an API reference generated by mkdocstrings from the Google-style docstrings (docs/api/*.md);
  • the README and CHANGELOG included through pymdownx.snippets, so each is written once.
• Absolute links: relative links in README.md, docs/CONTRIBUTING.md and docs/README.zh-TW.md become absolute GitHub URLs. Links inside an included snippet resolve against the page that includes them and are not checked by --strict, so relative ones broke on the site. Absolute links also work on PyPI.
• docs dependency group: zensical>=0.0.65,<0.1 and mkdocstrings-python. Zensical is pre-1.0, so upgrades should be deliberate. uv.lock gains only these packages.
• Same strict build in two places: .readthedocs.yaml and a new Docs workflow both run zensical build --strict, so a broken link or docstring reference fails the PR instead of the published site.
• [project.urls] gains a Documentation entry.

Notes

• site_url is fixed to https://fastapi-cachex.readthedocs.io/, because Zensical cannot read READTHEDOCS_CANONICAL_URL. It must match the Read the Docs project slug.
• CACHE_FLOW.md, JWT_CLAIMS.md and part of SESSION.md are in Chinese. They are published as-is; translating them is out of scope for Publish the documentation as a site on Read the Docs (Zensical) #81.

Verification

• zensical build --strict: no issues, including in a clean environment set up with uv sync --frozen --only-group docs --no-install-project, the same command CI and Read the Docs run.
• The API pages render: 15 class blocks on the Backends page.
• The pre-commit suite passes. The uv-lock hook was skipped, because it upgrades unrelated dependencies; uv lock --check passes.