Overhaul documentation

[Luthaf]

The documentation of rascaline is a bit lacking, and should be improved quite a lot. This will be a meta issue to list everything that needs to be done:

• re-organize the different pages to make navigation easier
• Document the convention used for metatensor labels #302
• Add a tutorial about "how to get fixed size output from rascaline" #310
• Add explanation section on how we compute radial integrals #299
• How-to on explaining what each calculator is doing #164
• Add back "SOAP hyper explanations" tutorial that was removed in Update hyper-parameters to a more extensible schema #328