Merge dataless

[pp-mo]

Closes #6740

[codecov]

Codecov Report

❌ Patch coverage is 97.43590% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.31%. Comparing base (7536010) to head (e0c2cd8).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/iris/cube.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6741      +/-   ##
==========================================
+ Coverage   90.29%   90.31%   +0.02%     
==========================================
  Files          91       91              
  Lines       24475    24493      +18     
  Branches     4571     4576       +5     
==========================================
+ Hits        22100    22122      +22     
+ Misses       1607     1605       -2     
+ Partials      768      766       -2     

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[pp-mo]

Status Update Fri 2023-10-10

I'm currently using this branch to add some more extensive documentation, since I felt I needed a decent location to describe the properties of merging with dataless cubes.

Also, I want to at least refer to #6739

Hence put this in draft for a while...