docs: add dark mode

[carueda]

This PR adds dark mode to the mkdocs-based documentation site. Pretty straightforward with the great mkdocs tooling. Default mode will reflect user's OS preference, with ability to toggle dark/light mode after that. Main adjustment only in mkdocs.yml, but I also adjusted one style in theme.css so the links in dark mode are a bit more legible.

[piksel]

Great idea! There really ought to be a containrrr-dark theme though, but it can be added in another PR.

[codecov]

Codecov Report

Base: 64.98% // Head: 64.98% // No change to project coverage 👍

Coverage data is based on head (aa01601) compared to base (1fc24f6).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1412   +/-   ##
=======================================
  Coverage   64.98%   64.98%           
=======================================
  Files          23       23           
  Lines        2319     2319           
=======================================
  Hits         1507     1507           
  Misses        714      714           
  Partials       98       98           

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[piksel]

The codacy/csslint error seems to be just wrong. Ignoring it.