[DOC] Update install.rst to reflect pyproject.toml

[JonathanShor]

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PR Description

Updates the Installation docs page to reflect the current pyproject.toml defined dependencies.

Merge checklist

Maintainer, please confirm the following before merging:

• [ ] All comments resolved
• [ ] This is not your own PR
• [ ] All CIs are happy
• [ ] PR title starts with [MRG]
• [ ] whats_new.rst is updated
• [ ] PR description includes phrase "closes <#issue-number>"

[adam2392]

I think the CI issue arises from sphinx version I guess?

@larsoner have you seen this error before?

[larsoner]

the short term solution is

https://github.com/mne-tools/mne-python/blob/156ae18572c8322ae75c964258f3fdc607006587/doc/conf.py#L809

the long term solution is

https://github.com/sphinx-gallery/sphinx-gallery/pull/1289

and other conf.py changes

[adam2392]

Thanks @JonathanShor !