Fix broken links in docs/contributing.md and CHANGELOG.md

[nick2i]

Summary

Similarly to #3123, some links to the /advanced/ section of the docs were out of date.
There is also one instance of link text changing from Writing custom transports -> Custom transports to match the section header.

There is an outstanding broken link in README.md that I wasn't sure about, maybe it should point to https://www.python-httpx.org/advanced/transports/#wsgi-transport?

[. . .] directly to [WSGI applications](https://www.python-httpx.org/advanced/#calling-into-python-web-apps) or [. . .]

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.

[tomchristie]

Thanks! ☺️