Restructure documentation

[PicoCentauri]

Based on my group meeting talk I restructured the docs for an better overview for new and experienced users and developers. There is mostly no new information added.

I am open for comments and suggestions.

[github-actions[bot]]

The documentation for this PR is (or will soon be) available on readthedocs: https://rascaline--112.org.readthedocs.build/en/112/

[Luthaf]

So I have a couple of changes to suggest. For me, the side bar should look something like this:

> Getting started
    > Installation
> How To
    > SOAP calculation
    > Sample selection (TBD)
    > Feature selection (TBD)
> Reference
    > Calculators hyper-parameters reference
    > API reference
        > Python
        > Rust
        > C
        > C++
> Explanations
    > Core concepts of rascaline (might also work in "getting started")
    > What is SOAP (TBD, similar to https://lab-cosmo.github.io/librascal/SOAP.html)
> Developer documentation
    > Getting started
        > Contributing
    > How To
        > Adding a new calculator
        > Profiling calculations
    > No dev-specific reference
    > Explanations
        > Code organization
        > Python and C interfaces

Some differences from the proposed organization in this PR:

• User doc is at the top level, no need to go inside a section to see them
• The API reference is targeted to users, so it should be in the user section of the docs
• We don't have dev specific reference material, so we can omit this section

[PicoCentauri]

I updated the structure accordingly. There a lot of TODOs and TBD in the docs but the at least the structure should be fixed now.

[PicoCentauri]

I am happy for now!