Update all .rst files for modules to include explanations of methods

[andrewtarzia]

• timesoap.rst
• soapify.rst
• lens.rst
• analysis.rst
• onion
• data_processing.rst
• hd5fer.rst

all in docs/source/

[matteobecchi]

I wrote the explanations in all the modules apart from data_processing and hdf5er (I never really understood/used the functions in those modules, and maybe in the future we are removing the hdf5er altogether...).

Should we already PR and merge these (partial) changes, so that everyone can see them and eventually copy the layout for what's missing, and external users have at least a bit of documentation?