Add coefficient_ring for modules and improve the docs for base_ring

[SoongNoonien]

This addresses #1207. Right now base_ring and coefficient_ring seem to be completely equivalent but with #2182 this will change. Depending on the merge order I can adjust this or the other PR to include an example (with universal polynomials) in the docs which clarifies the difference between base_ring and coefficient_ring.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.95%. Comparing base (10cb1ce) to head (614b51c).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/generic/QuotientModule.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2185      +/-   ##
==========================================
+ Coverage   87.93%   87.95%   +0.01%     
==========================================
  Files         127      127              
  Lines       31802    31795       -7     
==========================================
  Hits        27964    27964              
+ Misses       3838     3831       -7     

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[fingolfin]

Thanks for starting this! BTW I think we can also do something like this for series rings