Add dynamic colormap generation to docs

[djhoese]

This PR adds a custom sphinx directory so that anywhere in the docs you can do .. trollimage_colormap: trollimage.colormap.cmap_name (or point to a list or dict of colormaps) and it will make a list of the images. This way the colormaps on the site are always correct with what the code products.

I still have refactoring to do but the main idea is there.

• Closes #xxxx (remove if there is no corresponding issue, which should only be the case for minor changes)
• Tests added (for all bug fixes or enhancements)
• Tests passed (for all non-documentation changes)
• Passes git diff origin/master **/*py | flake8 --diff (remove if you did not edit any Python files)
• Fully documented (remove if this change should not be visible to users, e.g., if it is an internal clean-up, or if this is part of a larger project that will be documented later)

[codecov]

Codecov Report

Merging #137 (1e36aae) into main (3550c58) will increase coverage by 0.01%.
Report is 4 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #137      +/-   ##
==========================================
+ Coverage   91.17%   91.18%   +0.01%     
==========================================
  Files          11       11              
  Lines        3852     3847       -5     
==========================================
- Hits         3512     3508       -4     
+ Misses        340      339       -1     

Flag	Coverage Δ
unittests	91.18% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
trollimage/colormap.py	100.00% <100.00%> (ø)
trollimage/tests/test_colormap.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

[coveralls]

coverage: 91.374% (-0.05%) from 91.42% when pulling 6648200 on djhoese:feature-cmap-docs into 3550c58 on pytroll:main.

[mraspaud]

Very cool idea, looks great so far!

[djhoese]

Any last comments? Otherwise I'll merge this tomorrow.

[mraspaud]

LGTM!