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

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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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[blnicho]

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.

@ksbeattie readthedocs does not build any formats other than html for PRs (we just encountered this in Pyomo). You'll have to try building the pdf locally to check if this works or you can add a separate GHA job that builds the pdf (we did this in Pyomo here: Pyomo/pyomo#3495).

[lbianchi-lbl]

Since RTD will not pick up this change, maybe a good first step would be to try to run the build locally and see if it works or not at the sphinx-build level (?). Maybe @dangunter can look into that?

[ksbeattie]

I will try sphinx-build -M latexpdf . build and see if that works or not.

[ksbeattie]

After a sudo port install graphviz latexmk texlive texlive-latex-extra on my mac and running sphinx-build -M latexpdf . build I've gotten past the:

! LaTeX Error: Unicode character ↪ (U+21AA)
               not set up for use with LaTeX.

errors (by adding \DeclareUnicodeCharacter{21AA}{$\hookrightarrow$} to docs/conf.py) but I'm still getting:

! Emergency stop.
<inserted text>
                $
l.13337 \end{tabulary}

!  ==> Fatal error occurred, no output PDF file produced!

Not sure yet how to fix that.