Add documentation build steps, move doc dependencies into pyproject.toml

[N-Wouda]

Closes #142. This PR:

• Adds our own doc build process, so we can use poetry to manage the doc dependencies as well;
• Bumps the minimum ALNS Python version to 3.8;
• Adds Python 3.11 builds to the CI;

[codecov]

Codecov Report

Merging #143 (5fa8393) into master (f9159d2) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #143   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           28        28           
  Lines          668       668           
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  Hits           668       668           

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[N-Wouda]

@leonlan I don't think this needs a lot of motivation, but: I'm dropping Py3.7 from the CI and bumping our dependency list to minimally Py3.8. Py3.7 will be EOL in four months, and most other packages have already moved on (including some of the doc dependencies, so it's already not easy to develop ALNS on Py3.7).

This also clears the way for #61.