Add sphinx module

[tloubrieu-jpl]

💡 Description

For pds-api build to work (see action https://github.com/NASA-PDS/pds-api/actions/runs/4735783155/jobs/8406453520) We need the module sphinx_substitution_extensions to be added.

[tloubrieu-jpl]

@nutjob4life the github action still fails with the same error: https://github.com/NASA-PDS/pds-api/actions/runs/4885019123/jobs/8718534082

[nutjob4life]

@tloubrieu-jpl this is becoming a dependency nightmare:

• sphinx_substitution_extensions==2020.5.27.0 downgrades Sphinx from 3.2.1 → 3.0.4, which isn't ideal but at least is okay for now
• The next version up, sphinx_substitution_extensions==2020.5.30.0 upgrades Sphinx 3.2.1 → 6.2.1, which is totally unacceptable due to jinja2 conflicts
• The conf.py in pds-api is missing sphinx-prompt
• But adding sphinx-prompt then requires sphinx-copybutton==0.5.2 in github-actions-base

If we break up pds-github-util then all of this goes away because each repository can specify its own custom sphinx. (It would be better to spend my limited time on PDS on that!)

I'll have PRs ready to address these issues shortly.

[nutjob4life]

PRs are filed. Note the following must happen:

• [x] PR against pds-api must be merged
• [x] PR against github-actions-base must be merged
• [x] New :stable tag of the github-actions-base image in the Docker hub must be tagged after the second PR is merged and the new :latest image is published

[tloubrieu-jpl]

Thanks @nutjob4life , I think I already broke what you done by upgrading sphynx in the pds-api repo (because I was not able to generate the documentation locally anymore). Hopefull when I'll go across that again at the end of next build, pds-github-util will be broken up.