Document creating triangles with preexisting ultimates

[EKtheSage]

Summary

• Document how to create triangles with preexisting ultimates via options.ULT_VAL in the Triangle class docstring, with round-trip and direct-import examples
• Expand the _split_ult docstring to explain sentinel-date detection and the intended round-trip use case
• Add test_create_triangle_with_ultimates covering both patterns

Closes #523

Test plan

• pytest chainladder/core/tests/test_triangle.py::test_create_triangle_with_ultimates
• pytest chainladder/core/tests/test_triangle.py::test_create_full_triangle

---

Note

Low Risk
Documentation and regression tests only; triangle construction logic is unchanged aside from docstrings. The new workflow only pushes merges to a docs branch.

Overview
Documents how to build ultimate triangles using the sentinel valuation options.ULT_VAL: round-trip from reserving output via to_frame(keepdims=True) with development='valuation', or direct long-format import with valuation set to that date. The Triangle class docstring gains a new section with doctest examples; _split_ult’s docstring now describes sentinel detection and merge behavior (implementation unchanged).

Adds test_create_triangle_with_ultimates for round-trip equality with Chainladder().ultimate_ and for direct DataFrame construction with is_ultimate.

Also adds a GitHub Action that merges main into docs/great-docs-prototype on every push to main.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.96%. Comparing base (5c1fc23) to head (992622e).
⚠️ Report is 308 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #968      +/-   ##
==========================================
+ Coverage   86.13%   88.96%   +2.82%     
==========================================
  Files          86       89       +3     
  Lines        4941     5091     +150     
  Branches      643      655      +12     
==========================================
+ Hits         4256     4529     +273     
+ Misses        486      416      -70     
+ Partials      199      146      -53     

Flag	Coverage Δ
unittests	88.96% <100.00%> (+2.82%)	⬆️

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