Improve documentation

[RaulPPelaez]

• [x] Added more information to the documentation of some classes.
• [x] Also added some details to the help you get for charge/spin when running torchmd-train --help
• [x] Fixed black formatting in some files.
• [x] Add a readthedocs webpage

[RaulPPelaez]

I went ahead and started integrating a Sphinx documentation to publish it in https://docs.readthedocs.io/en/stable/ Luckily, this being a python project makes it possible to easily autogenerate most of it from the docstrings. Alas, it requires being careful with the docstring format so sphinx understands it.

Thus far I have a component tree: And doc pages for some of the main classes/functions, which look like this:

[RaulPPelaez]

This is ready to merge. @guillemsimeon @AntonioMirarchi @raimis @giadefa Any comments about how the docs are set up now? The docs in this PR are (and will be) hosted here https://torchmd-net.readthedocs.io

• After this is merged ReadTheDocs will pick up on any new release tag and generate a new version automatically, by default the latest one is showed.
• I added a github workflow to check that the docs are building (this is all it does).

If you are ok with it please review/approve the PR so I can merge. In any case comments/suggestion are appreciated.