Add charts for colordiff to docs and Use docstrings for metrics

[kimikage]

This update the docs for color differences (cf. #398 (comment)).

I changed the example for colordiff, i.e. removed the strange parse.

I also add a doctest for the difference between an HSV color and an HSL color. It may help check the PR #407. The PR #407 is ready for merging, but there may be room for improvement on the fma/muladd (cf. #407 (comment)). So, it is another reasonable option to merge this PR first. (Edit: Either is fine now.)
PR #407 was merged.

[codecov]

Codecov Report

Merging #409 into master will increase coverage by 0.38%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #409      +/-   ##
==========================================
+ Coverage   80.58%   80.96%   +0.38%     
==========================================
  Files          11       11              
  Lines         896      893       -3     
==========================================
+ Hits          722      723       +1     
+ Misses        174      170       -4     

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[kimikage]

The changes of the constructors in this PR do not substantially break the functionality. Corrections and improvements of the docs are always welcome.