Integrate documentation for free associative algebras / non-commutative GB from AbstractAlgebra into OSCAR manual

[fingolfin]

We have functionality for free associative algebras and non-commutative Gröbner bases in AbstractAlgebra, but if one goes to the relevant section in the OSCAR manual there is very little functionality visible.

But there is documentation for these features, it just is in the AA manual. Which of course is included in the OSCAR manual, so one can find it using the search feature, if one knows what to search for, at https://docs.oscar-system.org/stable/AbstractAlgebra/free_associative_algebra/ (and it will be slightly improved in the next update, see this PR for AA).

We should make sure this information is also visible to OSCAR users. E.g. by adding a link in the former page to the latter; or copying the contents from the AA page to the OSCAR page; or something else, to be determined.