Add CI step to build the docs

[backmari]

Description of work:

Before the move to GitHub we had a CI step to build the docs. It is useful to get feedback on the docs builds, so let's add it as a step in the CI.

This does not require a release note since it only affects the CI pipeline.

Check all that apply:

• added release notes (if not, provide an explanation in the work description)
• updated documentation
• Source added/refactored
• Added unit tests
• Added integration tests
• Verified that tests requiring the /SNS and /HFIR filesystems pass without fail

References:

• Links to IBM EWM items:
• Links to related issues or pull requests:

Manual test for the reviewer

Check list for the reviewer

• release notes updated, or an explanation is provided as to why release notes are unnecessary
• best software practices
  • clearly named variables (better to be verbose in variable names)
  • code comments explaining the intent of code blocks
• All the tests are passing
• The documentation is up to date
• code comments added when explaining intent

Execution of tests requiring the /SNS and /HFIR filesystems

It is strongly encouraged that the reviewer runs the following tests in their local machine
because these tests are not run by the GitLab CI. It is assumed that the reviewer has the /SNS and /HFIR filesystems
remotely mounted in their machine.

cd /path/to/my/local/drtsans/repo/
git fetch origin merge-requests/<MERGE_REQUEST_NUMBER>/head:mr<MERGE_REQUEST_NUMBER>
git switch mr<MERGE_REQUEST_NUMBER>
conda activate <my_drtsans_dev_environment>
pytest -m mount_eqsans ./tests/unit/ ./tests/integration/

In the above code snippet, substitute <MERGE_REQUEST_NUMBER> for the actual merge request number. Also substitute
<my_drtsans_dev_environment> with the name of the conda environment you use for development. It is critical that
you have installed the repo in this conda environment in editable mode with pip install -e . or conda develop .

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.18%. Comparing base (d7cab0e) to head (1810577).

Additional details and impacted files

@@           Coverage Diff           @@
##             next     #995   +/-   ##
=======================================
  Coverage   86.18%   86.18%           
=======================================
  Files          98       98           
  Lines       10760    10760           
=======================================
  Hits         9274     9274           
  Misses       1486     1486           

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