Finalize sub-plot API and add documentation

[mattpap]

Early work in progress. First I need to finalize PR #12068 and then I will complete the documentation here.

• fixes Add user guide section about sub-coordinates/subplots #12073

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 92.45%. Comparing base (ed5f22b) to head (7b7a07e).
Report is 296 commits behind head on branch-3.5.

Additional details and impacted files

@@             Coverage Diff             @@
##           branch-3.5   #13346   +/-   ##
===========================================
  Coverage       92.45%   92.45%           
===========================================
  Files             315      315           
  Lines           20169    20173    +4     
===========================================
+ Hits            18647    18651    +4     
  Misses           1522     1522           

[bryevdv]

@mattpap Is this where you want to hash out x_source / x_target? I do think reusing "source" in this different context is unadvised.

[mattpap]

Closing this in favour of changes in PR #13344.