Update docs references from httpx to httpx2

[cclauss]

Fixes: https://httpx2.pydantic.dev/quickstart etc.

Summary

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

Create a pull request that modifies files in the docs/ directory (and its subdirectories) to change httpx to httpx2. In these files:

• Change any dependency mentions from httpx to httpx2.
• Change all imports such as import httpx to import httpx2.
• Change all from imports such as from httpx import xyz to from httpx2 import xyz.
• etc.

In the modified files, please replace URLs containing https://github.com/encode/httpx/ with https://github.com/pydantic/httpx2/. Test each modified URL, and if the resource is unreachable, then revert to the original URL that starts with https://github.com/encode/httpx/

The file docs/third_party_packages.md is not ready, so please do:
git checkout main -- docs/third_party_packages.md

Agent-Logs-Url: https://github.com/cclauss/httpx2/sessions/9a801e7e-2c67-43e4-b70e-3c914523170d

[codspeed-hq]

Merging this PR will not alter performance

✅ 7 untouched benchmarks

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_{Comparing cclauss:docs-httpx-to-httpx2 (bb81c59) with main (b4c5940)}

[Kludex]

Thanks so much!