Use readthedocs CI for building documentation

[JCGoran]

RTD supports PR builds: https://docs.readthedocs.com/platform/stable/guides/pull-requests.html
so we may as well use it instead of the github action for the docs.

This brings two advantages:

• we don't have 2 CIs that do the same thing
• we actually get automatic feedback when the documentation fails to build and deploy; currently we need to go through the tedious process of enabling each build by hand on RTD to see whether it works properly

Note that this is based on #3327 because strictly speaking the two PRs are logically separate, but if needed one can just merge this one.

[JCGoran]

You can see the docs for this PR on https://nrn--3328.org.readthedocs.build/en/3328/ (pattern appears to be https://nrn--[PR_NUMBER].org.readthedocs.build/en/[PR_NUMBER]/; you can also click on "Details" next to the docs/readthedocs.org:nrn CI job).

@pramodk @nrnhines to merge this, we would need to disable the requirement for the Documentation CI since I don't have permissions to do this, and enable it for docs/readthedocs.org:nrn (or whatever it's called in the settings).

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[nrnhines]

@pramodk @nrnhines to merge this, we would need to disable the requirement for the Documentation CI since I don't have permissions to do this, and enable it for docs/readthedocs.org:nrn (or whatever it's called in the settings).

I'm happy to do that. Just want to ask if the only (slightly) negative consequence is that for a release, it is necessary to manually verify that the readthedocs successfully concluded. (Or does the github ci still show a failure but it does not prevent merging?)

Nevermind. There appears to be no negative consequence. You noted "we actually get automatic feedback when the documentation fails to build and deploy" and I see in the CI list

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.08%. Comparing base (df306bd) to head (a1a1951).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3328   +/-   ##
=======================================
  Coverage   67.08%   67.08%           
=======================================
  Files         571      571           
  Lines      111039   111039           
=======================================
  Hits        74485    74485           
  Misses      36554    36554           

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[azure-pipelines]

✔️ a1a1951 -> Azure artifacts URL