Add documentation for self-referential extras (#11296)

[yahya-mhf]

Summary

Adds documentation for self-referential extras, allowing users to understand
how to declare optional dependencies that reference other optional dependencies
in the same project.

Changes

• Added docs/html/topics/self-referential-extras.md with an example using [project.optional-dependencies].

Related issue

Fixes #11296

[sepehr-rs]

Hi @yahya-mhf, thank you for your interest in contributing to pip!
I noticed a couple of issues with your PR:

1. Based on pip's contribution guidelines, your PR needs a news entry file.
2. There are a few issues with your documentation, which cause the tests to fail. You can find more info here.

Please let me know if you need any help in fixing them!

[notatallshaw]

I don't think this justifies having it's own page: https://github.com/pypa/pip/blob/f2ab7e3f6cf3b6a66e0f0e0a604e255d1f52442f/docs/html/topics/self-referential-extras.md. It should be somewhere else, perhaps the user guide unless there's somewhere better.