docs(python): publish preview build contract

[Windsor Nguyễn (windsornguyen)]

What does this change?

• Adds a Python ecosystem page to the MkDocs navigation and LLM documentation index.
• Documents the zero-configuration PEP 751 project contract and generated targets.
• Makes native Astral configuration authoritative instead of introducing a parallel BSMR DSL.
• Separates the verified preview surface from the unimplemented RFC 0004 release gates.
• States the recommended PEP 517 migration policy and rejects silent legacy fallback.

The page is intentionally based on main so the documentation-site change remains independent of the implementation stack in #57 and #83–#95.

How was it tested?

• uv run --isolated --with-requirements docs/requirements.txt mkdocs build --strict

• pnpm run ci check license

• git diff --check

• CLA/Vouch check passes, or this PR only updates VOUCHED.td