Docs about custom coordinates

[astrofrog]

Fixes #1993

[codecov]

Codecov Report

Merging #1994 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1994      +/-   ##
==========================================
- Coverage   87.76%   87.74%   -0.03%     
==========================================
  Files         248      248              
  Lines       21684    21684              
==========================================
- Hits        19031    19026       -5     
- Misses       2653     2658       +5

Impacted Files	Coverage Δ
glue/app/qt/application.py	77.93% <0%> (-0.84%)	⬇️
glue/core/data_factories/fits.py	88.32% <0%> (+0.72%)	⬆️
glue/utils/qt/helpers.py	55.55% <0%> (+1.11%)	⬆️

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