Dependencies list is duplicated between documentation and requirements.txt

[Luthaf]

The dependencies required to build documentation are duplicated between the documentation itself and the requirements.txt file. We should point users to the later as much as possible to remove duplication. We could even split documentation requirements to docs/requirements.txt, and use -r docs/requirements.txt in the root requirements.txt to keep the current behavior.

Originally posted by @Luthaf in https://github.com/cosmo-epfl/librascal/pull/307#discussion_r602265858