docs: add Triangle.heatmap examples (#704)

[EKtheSage]

Summary of Changes

• Add doctest Examples for Triangle.heatmap in chainladder/core/display.py.
• Cover the default colormap, an alternate cmap, and the multi-dimensional ValueError.

Related GitHub Issue(s)

• API Reference Examples #704

Additional Context for Reviewers

Bite-sized core-module follow-up for #704. Companions: slicing #1208, I/O #1209.

HTML output is asserted by type and by the presence of background-color rather than dumping the full markup.

Checklist

• I passed tests locally for both code (uv run pytest) and documentation changes (uv run --directory docs jb build . --builder=custom --custom-builder=doctest)

Made with Cursor

[github-actions]

Pyright Type Completeness

View the full pyright --verifytypes output for this commit

Project (full chainladder package, at this PR's head): 15.1% of exported symbols fully typed (201 / 1333)

	Known	Ambiguous	Unknown	Total
Project (head)	201	111	1021	1333

Other symbols referenced but not exported by chainladder: 13

	Known	Ambiguous	Unknown	Total
Other (head)	3	1	9	13

Symbols without documentation:

• Functions without docstring: 321
• Functions without default param: 0
• Classes without docstring: 10

Patch (exported symbols added or changed by this PR): no exported symbol type-completeness changes detected.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.26%. Comparing base (52e655b) to head (1ff506c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1210   +/-   ##
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  Coverage   91.26%   91.26%           
=======================================
  Files          91       91           
  Lines        5401     5401           
  Branches      691      691           
=======================================
  Hits         4929     4929           
  Misses        338      338           
  Partials      134      134           

Flag	Coverage Δ
unittests	91.26% <ø> (ø)

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[henrydingliu]

I think the average user will want to see actual heatmaps. It would be more valuable to not add any examples in the docstring instead, just a link to a notebook in the docs that actually have heatmap.

[EKtheSage]

Dropped the HTML doctests. heatmap returns IPython HTML, so those examples could not show the gradient.

The docstring now points at the existing rendered example in the Triangle user guide (triangle.link_ratio.heatmap()).

[henrydingliu]

validated link via PR build

[henrydingliu]

@EKtheSage thanks!