BUG: Add horizontal_datum_name for geographic CRS in CRS.to_cf

[snowman2]

• Closes horizontal_datum_name missing from the CF version of a CRS, breaking conformity with CF ≥ 1.7 #1251
• Tests added
• Fully documented, including history.rst for all changes and api/*.rst for new API

[codecov]

Codecov Report

Merging #1255 (9405668) into main (8109fc4) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1255      +/-   ##
==========================================
+ Coverage   96.34%   96.40%   +0.06%     
==========================================
  Files          20       20              
  Lines        1805     1810       +5     
==========================================
+ Hits         1739     1745       +6     
+ Misses         66       65       -1     

Impacted Files	Coverage Δ
pyproj/crs/_cf1x8.py	98.07% <100.00%> (+0.69%)	⬆️
pyproj/crs/crs.py	98.08% <100.00%> (+<0.01%)	⬆️

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