Upgrade Documenter to 0.27

[mortenpi]

The documentation has not deployed for nearly 3 years. It looks like it tries to use some TRAVIS_* variables, because the ancient Documenter version that it tries to use probably predated full GitHub Actions support. I hope that bumping Documenter is enough (fix #397).

I checked the built docs manually, and I didn't notice any major issues with the upgrade.

The only thing is that it looks like the doctests are pretty broken in general. I don't quite understand why the current CI is passing, but I assume that it will start failing now. But there have also been changes in Documenter, so it's probably a multitude of factors here.

A few changes in general:

• Decouples the documentation CI from doctests. I'd say you don't want the docs for a tag to fail to deploy just because you happen to have one doctest that is outdated.
• Just doctest on 1.9, because printing is not stable across Julia versions anyway, so doctesting with different Julia versions is not really recommended. You should target a specific version. Also, since we're committing the manifest here, we don't run into any strange problems with that -- the manifest is from 1.9 now.
• Removes the Pkg.develop(PackageSpec(path=pwd())) lines (from CI), since those are not necessary when you have the Manifest anyway.

[mortenpi]

I would appreciate if someone could approve the CI run so that I could see if the changes are working or not.

[omus]

Thanks @mortenpi! I appreciate the PR

[codecov-commenter]

Codecov Report

Merging #436 (472c6f0) into master (3bdc66d) will not change coverage. The diff coverage is n/a.

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@@           Coverage Diff           @@
##           master     #436   +/-   ##
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  Coverage   95.57%   95.57%           
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  Files          38       38           
  Lines        1762     1762           
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  Hits         1684     1684           
  Misses         78       78