CHORE: Update workflows to test docs examples.

[genedan]

This PR adds a workflow to run cd docs && uv run jb build . --builder=custom --custom-builder=doctest, which runs unit tests found within docstrings.

If successful, we should see a failure in the checks regarding Voting Chainladder. When running locally, doctest found an error in an example there, so I'd expect to see it here.

@kennethshsu, do you know what Python version the readthedocs is running? I can update the workflow to reflect the correct version.

---

Note

Low Risk
Low risk: changes are limited to CI and documentation doctest formatting, with no runtime/library logic modifications. Main risk is potential CI instability if doctest outputs vary across environments or versions.

Overview
Adds a new GitHub Actions doctest job that installs the docs extras and runs jupyter-book with a custom doctest builder, so doc examples are executed in CI.

Enables sphinx.ext.doctest in the docs config and rewrites select docstring examples (e.g., Triangle and VotingChainladder) into explicit .. testsetup::/.. testcode:: blocks with captured .. testoutput::, making their printed outputs part of the test surface.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.12%. Comparing base (6ea3577) to head (be1125a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #713   +/-   ##
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  Coverage   85.12%   85.12%           
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  Files          85       85           
  Lines        4909     4909           
  Branches      629      629           
=======================================
  Hits         4179     4179           
  Misses        521      521           
  Partials      209      209           

Flag	Coverage Δ
unittests	85.12% <ø> (ø)

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[kennethshsu]

No I do not, I think @jbogaardt is the only one with access to manage the doc site.

[kennethshsu]

And sorry for going back and forth here, are we deciding to go this route? Instead of keeping the imports, or should we continue the discussion in #704?

[genedan]

I first intended for this to just include the new worfklow, but then it wouldn't pass because there are pre-existing errors that I had to patch. Unfortunately this will create merge conflict for #714. Once we come to a decision in #704, I can update this PR, and then @EKtheSage can update his.

[genedan]

@kennethshsu, I updated the voting chainladder example to match the format we decided upon yesterday. This PR is ready for review.

[jbogaardt]

I've sent an invite to @genedan to join the readthedocs admin. Kenneth, it says your email isnt found so I assume you dont have a registered userid for readthedocs. Gene let me know if the invite doesn't work for whatever reason.