MNT - remove requirements files

[Badr-MOUFAD]

Now that we switched to pyproject.toml, it is possible to centralize all the repo requirements in one place.

This removes:

• [x] requirements.txt
• [x] doc-requirements.txt and addresses all the implied sides effects

---

followup of #284

[codecov-commenter]

Codecov Report

Merging #285 (e66278f) into main (922f7cc) will not change coverage. The diff coverage is n/a.

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Additional details and impacted files

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