Add PDF generation by ReadTheDocs

[ksbeattie]

Fixes

The lack of PDF version of our docs at RTD

Summary/Motivation:

This is a test to see if we want to have RTD produce a PDF version of the docs

Changes proposed in this PR:

• add format: pdf to .readthedocs.yml

Legal Acknowledgement

By contributing to this software project, I agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the license terms described in the LICENSE.txt file at the top level of this directory.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[ksbeattie]

I expected a PDF option when going to the flyout menu at the bottom of the page. It isn't there, not sure why not.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.86%. Comparing base (5f3b33b) to head (9a04e4d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1582      +/-   ##
==========================================
- Coverage   76.86%   76.86%   -0.01%     
==========================================
  Files         394      394              
  Lines       63233    63233              
  Branches    10355    10355              
==========================================
- Hits        48604    48601       -3     
- Misses      12184    12188       +4     
+ Partials     2445     2444       -1     

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