Deploy httpcore2 docs as a standalone site

[Kludex]

Builds the httpcore2 docs as their own site at httpcore2.pydantic.dev, with its own objects.inv. The previous standalone src/httpcore2/mkdocs.yml was never wired into the build and still pointed at encode/httpcore.

• Repoint src/httpcore2/mkdocs.yml at pydantic/httpx2 and align its mkdocstrings/theme/markdown config with the httpx2 site, so the ::: httpcore2.X API references render.
• Rewrite httpcore -> httpcore2 across the docs and repoint badge/PyPI/GitHub URLs.
• Add a second zensical build step and a wrangler-httpcore2.toml Worker; deploy + PR-preview jobs mirror the httpx2 ones.

Verified locally: both sites build with no unresolved references, and httpcore2.pydantic.dev/objects.inv is emitted with proper httpcore2.* symbols (httpcore2.ConnectionPool, httpcore2.Proxy, etc.).

Supersedes #997 (which merged the docs into the httpx2 site under a shared objects.inv).

Note: httpcore2.pydantic.dev must be added as a custom domain in Cloudflare for the route to resolve.

AI Disclaimer

This PR was developed with the assistance of either Claude or Codex. I've reviewed and verified the changes.

[github-actions]

Docs preview: https://15ef8b18-httpx2-docs.pydantic.workers.dev
httpcore2 docs preview:

[codspeed-hq]

Merging this PR will not alter performance

✅ 15 untouched benchmarks
⏩ 7 skipped benchmarks¹

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_{Comparing httpcore2-docs-site (09897f9) with main (999da32)}

Footnotes

1. 7 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Don't expose the unfinished requests page

Adding requests-responses-urls.md to the standalone nav makes this page part of the published httpcore2 docs, but the page still has an unclosed Python fence starting at src/httpcore2/docs/requests-responses-urls.md:25; as a result everything from “Responses” through the ::: httpcore2.Request/Response/URL directives is rendered as literal code instead of prose/API reference. Either close/fix that page before linking it or leave it out of the nav so the new standalone site doesn't publish a broken API page.

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[Kludex]

I think this is a bit too much. We don't need 2 docs.