Add tox environment to enable local docs builds

[haticekaratay]

Description

This pull request introduces a new tox env to build and test the docs locally. After merging, contributors can run:

tox -e docs

This command builds the HTML docs locally for review.

Fixes #

Change log entry

• Is a change log needed? If yes, is it added to CHANGES.rst? If you want to avoid merge conflicts,
  list the proposed change log here for review and add to CHANGES.rst before merge. If no, maintainer
  should add a no-changelog-entry-needed label.

Checklist for package maintainer(s)

This checklist is meant to remind the package maintainer(s) who will review this pull request of some common things to look for. This list is not exhaustive.

• Are two approvals required? Branch protection rule does not check for the second approval. If a second approval is not necessary, please apply the trivial label.
• Do the proposed changes actually accomplish desired goals? Also manually run the affected example notebooks, if necessary.
• Do the proposed changes follow the STScI Style Guides?
• Are tests added/updated as required? If so, do they follow the STScI Style Guides?
• Are docs added/updated as required? If so, do they follow the STScI Style Guides?
• Did the CI pass? If not, are the failures related?
• Is a milestone set? Set this to bugfix milestone if this is a bug fix and needs to be released ASAP; otherwise, set this to the next major release milestone. Bugfix milestone also needs an accompanying backport label.
• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)?

[pllim]

But why not run

cd docs
make html

?

I think that is basically what RTD is doing. Does not hurt to add this but it is really untested in CI because we use RTD PR builder so it could rot silently.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.81%. Comparing base (7e5ddfa) to head (9806695).
Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3314   +/-   ##
=======================================
  Coverage   88.81%   88.81%           
=======================================
  Files         125      125           
  Lines       19030    19036    +6     
=======================================
+ Hits        16901    16907    +6     
  Misses       2129     2129           

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[haticekaratay]

But why not run

cd docs
make html

?

I think that is basically what RTD is doing. Does not hurt to add this but it is really untested in CI because we use RTD PR builder so it could rot silently.

Oh, okay, I see your point about the redundancy and the risk of this tox env going stale. I'll go ahead and close this PR. Thanks again for your input :)

[pllim]

FWIW astropy does have something like this but there is really no way to know if it still works until someone actually uses it. And I have been fighting PRs to add that to the CI because it lengthens the run time (astropy docs take a while to build) while duplicating RTD run.

https://github.com/astropy/astropy/blob/1782ff94ce2c19c2f5d3e2f5b32bd11800cbc98d/tox.ini#L132-L134