Moving docs' requirements into the actual docs' requirements file

[ghost]

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This PR moves the specific requirements of the documentation (matplotlib and gitpython) which are currently in the setup.py file to the docs/requirements.txt file, which is more appropriate as they are not used elsewhere.

[maikia]

@mandreux-owkin thanks for your contribution!
the cleaning up is a great idea, and it's true that gitpython and matplotlib should not be in setup.py if they are only used for docs.
but here it might be a bit more complex:
Sphinx is generating the docs, but we also use binder to run the example notebooks. (refer to .mybinder directory) There, in postbuild we are actually uninstalling all the docs/requirements to have a lighter build. I am afraid with this change the binder won't work. Feel free to suggest alternative solution.

[ghost]

@maikia thanks for your careful review. I checked the notebooks code and I am certain that gitpython is not used. We don't use any plot in the jupyter but I added matplotlib again to avoid breaking binder

[maikia]

LGTM, thanks @mandreux-owkin