docs: Adding documentation for this library

[gkrajniak]

This change adds the first piece of documentation for end users.
It contains a getting started part describing all the portal module options to consider.
There is also a part added on local development.
Lastly, there is a documentation on the contents of a content configuration file added

[vzipf]

@gkrajniak and @rrogoda thank you very much for this big chunk of work.
I allowed myself to extend some explanations and fix wording here and there.

I think we are mostly done here.
We just need to make sure that we do not refer to any internal knowledge or unpublished material. Otherwise - if this repo will be public - people may be confused

[vzipf]

Lets merge this big chunk of information and work in the detail improvements in follow ups.
Great work, rly!