Combined Documentation is not very visible on Github

[tramsauer]

Only https://multiply-core.readthedocs.io/en/doc/ holds the combined documentation which in my opinion definitely should to be on the "home page"/master page for users searching for documentation (supporting the test users?).
Therefore, I suggest adding the link or better merging the efforts from https://github.com/multiply-org/multiply-core/tree/doc
@TonioF ? @JorisTimmermans

[tramsauer]

to have a cleaner 'Components' section in the core docs, SIAC (@MarcYin ) and gp_emulators (@jgomezdans ) could add a top headline so that 'indices and tables' does not show up for each of the repositories in the listing.

(The list of components is retrieved via:

:maxdepth: 1
   :caption: Contents:

   data-access/doc/source/index
   atmospheric_correction/docs/source/index
   gp_emulator/docs/index
   ...

)