Document self-referential extras now that pip officially supports it

[Trenza1ore]

Issue Description

Following pypa/pip#14157, self-referential extras is now a documented and tested feature of pip, which along with uv have supported this feature for years. This is a more preferable pattern than to manually write, for example, an all extra for all dependencies combined (which is both tedious and easy to fall out of sync with reality).

Code of Conduct

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