How-to on explaining what each calculator is doing

[PicoCentauri]

Each of the calculators has a short docstring, but all of them are missing an example. Also, the new AtomicComposition one has this issue pointed out by @frostedoyster in a comment of the PR.

This is a valid point because the lack of examples makes it harder for new users to apply this. But, specific examples in the docstring of every calculator is maybe not the best idea for us. We have to write these examples in four languages and they are more complex than a couple of lines: You need to create/load a structure, define the hypers, do the computation, and look at the results, ...

Instead of putting an example to each of them, we could do one how-to guide (or one for each calculator) similar to Understanding SOAP hyperparameters.

In this how-to guide, we explain the idea of the calculators, link to publications, and apply each of them to the same dataset. As a side note, there is a package called sphinxcontrib-bibtex which would allow us to create a use bibliography for this.