Add missing Abelian groups docs

[lgoettgens]

Similarly to the update in Hecke:
thofma/Hecke.jl#1631

(this is a copy of #4184, but this time not from a fork to get a preview)

preview: https://docs.oscar-system.org/previews/PR4186

[lgoettgens]

This is still broken, since I just cherry-picked the branch:

[lgoettgens]

I think the change in 4e95637 (#4186) makes it behave like in the Hecke navigation bar.

cc @paemurru

[lgoettgens]

A local build now looks like this:

The preview should look the same (once the CI job finishes).

Is this what you imagined @paemurru ?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.58%. Comparing base (4d1d833) to head (4e95637).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4186      +/-   ##
==========================================
- Coverage   84.59%   84.58%   -0.01%     
==========================================
  Files         631      631              
  Lines       84829    84829              
==========================================
- Hits        71758    71756       -2     
- Misses      13071    13073       +2     

see 1 file with indirect coverage changes