Sync notebook github links in HTML docs to the git commit used to build them

[bmorris3]

Description

The HTML docs on RTD like this page point to directories on github like https://github.com/spacetelescope/jdaviz/tree/main/notebooks. We've changed the default RTD page to stable, which isn't the same as main, so the notebooks linked on RTD will be out of sync with the rest of the HTML docs whenever main != stable.

This PR gets the git commit hash at the time the docs are built, and injects that hash into the github URLs for docs links.

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)? 🐱

[bmorris3]

In the RTD build for this PR, you can prove to yourself that this is working by trying out the following links and comparing the commit hash to the most recent commit hash in this PR:

• in Quickstart, click the link to ImvizExample.ipynb under the first subheading "In a Jupyter Notebook"
• from the landing page, click "Sample Notebooks" > "Sample Jdaviz Notebooks"

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.79%. Comparing base (e65237f) to head (2201752).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3080   +/-   ##
=======================================
  Coverage   88.79%   88.79%           
=======================================
  Files         111      111           
  Lines       17236    17236           
=======================================
  Hits        15305    15305           
  Misses       1931     1931           

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[pllim]

Simpler than what I thought it would be. Seems to work and RTD is happy. Thanks!