Add color inversion for docs images in dark themes

[leburgel]

I was playing around with automatically inverting image colors based on the selected documenter theme, and thought TensorKit.jl could benefit from this. I also added a docs preview cleanup action to remove the docs preview after a PR is merged.

Fixes #128.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.08%. Comparing base (ffa6cf9) to head (5fea1dc).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #253   +/-   ##
=======================================
  Coverage   83.08%   83.08%           
=======================================
  Files          44       44           
  Lines        5647     5647           
=======================================
  Hits         4692     4692           
  Misses        955      955           

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[leburgel]

It seems that the docs preview doesn't build for a PR from a fork, which is a bit annoying. Everything builds nicely locally, but I don't know how to force the preview build so we could check...

[lkdvos]

I don't think there is a way to get that to work actually, but if it builds nicely locally we can just try merging and seeing what happens. @Jutho , is that okay for you?

[leburgel]

Some "proof":

[VictorVanthilt]

🙏 🙏 🙏