Document composites

[pnuu]

This a second try in the PR on adding documentation on composites. The first time (#513) got messed up with confusement on PR branch's location on my part. This should be cleaner and should contain all suggestions made in the first attempt.

• Closes Add/update documentation about composites and compositors #511

[codecov]

Codecov Report

Merging #705 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #705      +/-   ##
==========================================
+ Coverage   79.68%   79.68%   +<.01%     
==========================================
  Files         145      145              
  Lines       21192    21192              
==========================================
+ Hits        16886    16887       +1     
+ Misses       4306     4305       -1

Impacted Files	Coverage Δ
satpy/composites/__init__.py	65.55% <100%> (ø)	⬆️
satpy/scene.py	89.9% <0%> (+0.18%)	⬆️

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[coveralls]

Coverage remained the same at 79.681% when pulling d1e97f4 on pnuu:document-composites into 6e76f37 on pytroll:master.