put intro of scikit-matter into the index page and abbreviate
add getting started page which gives an overview of important implementations
include existing introductory text for reconstruction measures into API reference so all introductory texts from the API are included into the getting started
reword text a bit for more soundness within the getting started
:books: Documentation preview :books:: https://scikit-matter--202.org.readthedocs.build/en/202/