Make docs/ a pure Markdown content directory

[kyleconroy]

First step toward moving the documentation site off Read the Docs: docs/ becomes a plain Markdown content directory with no rendering toolchain in this repo. A separate repo will consume it and build the website.

Changes

• Remove all Read the Docs / Sphinx configuration: .readthedocs.yaml, docs/conf.py, docs/requirements.txt, docs/Makefile, the RTD theme templates in docs/_templates/, and docs/_static/. Also removes the pip ecosystem for /docs from dependabot and the .readthedocs.yaml entries from workflow paths-ignore lists.
• Port the three remaining .rst files to Markdown: index.rst → index.md, reference/language-support.rst → .md (rst grid tables → GFM tables), guides/using-go-and-pgx.rst → .md. Admonitions use GitHub alert syntax (> [!NOTE]), which renders natively on github.com and stays a plain blockquote for any CommonMark parser. The three tutorial links that pointed at language-support.rst now point at the .md file.
• Add docs/toc.yaml: the navigation order previously encoded in index.rst's toctree directives. Every page appears exactly once — in a titled section, or in unlisted (published but not in the sidebar, matching the three migration guides' current behavior). Verified 1:1 against the 41 files on disk.

Follow-ups (not in this PR)

• A Go-based linter (goldmark) run from go test that validates the content contract: links resolve, every page is in toc.yaml, no raw HTML/MDX.
• The consuming repo that builds and deploys the site.
• The remaining MyST admonitions (:::{note} etc.) in a few howto pages can be converted to GitHub alert syntax when the linter lands.

🤖 Generated with Claude Code

https://claude.ai/code/session_013XDcHxZk9UzfzMLayG66Zq

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