Documentation core folder

[Christian48596]

Done all documentation inside the folder 'core'

[ilfreddy]

Thanks for the documentation work!
A couple of comments:

1. a good portion of it seems to be not in doxygen format. Do you think you can make it doxygen-friendly?
2. we have had a discussion lately on Zulip on whether documentation should go in the header files. We have not reached a consensus yet. So I would like to wait with this merge until that is decided.

[ilfreddy]

Additional comments:

1. the documentation builds fails which is maybe defeats the point of a documentation PR? 😉
2. Conflicts must be solved

[Christian48596]

I have solved the conflicts now looking for the compilation

[Christian48596]

Now fixing the conflicts it is doxygen compatible

[Christian48596]

All checks passed!

[ilfreddy]

I think this could be an excellent starting point for a "docathon" where we go through this autogenerated documentation and make it more human-readable.

[ilfreddy]

Superseded by #265