Add documentation for ppc_data and ppc_loo_pit_data

[utkarshpawade]

Fixes #209

Changes

• Added documentation for ppc_data() in PPC-distributions help page
• Added documentation for ppc_loo_pit_data() in PPC-loo help page
• Both functions are now documented as data-preparation back ends
• Users can call them directly to create custom plots with ggplot2
• Updated NEWS.md with user-facing change

Testing

• Both functions tested and working correctly
• Documentation verified in help pages
• Custom plotting example confirmed working

[Copilot]

Pull request overview

Adds missing user-facing documentation for two exported PPC data-preparation helpers (ppc_data() and ppc_loo_pit_data()), clarifying that they can be called directly to build custom ggplot2 visualizations.

Changes:

• Documented ppc_loo_pit_data() in the LOO PPC help page and updated the return-value template to cover _data() helpers.
• Documented ppc_data() in the PPC distributions help page as a data-preparation helper.
• Added a NEWS entry noting the new documentation.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
man/PPC-loo.Rd	Updates value text and adds a Plot Descriptions entry for ppc_loo_pit_data().
man/PPC-distributions.Rd	Adds a Plot Descriptions entry for ppc_data().
R/ppc-loo.R	Switches to @template return-ggplot-or-data and adds roxygen text for ppc_loo_pit_data().
R/ppc-distributions.R	Adds roxygen text describing ppc_data() as a plotting data helper.
NEWS.md	Adds a bullet noting documentation for ppc_data() / ppc_loo_pit_data().

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.63%. Comparing base (93cc02d) to head (7195e72).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #426   +/-   ##
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  Coverage   98.63%   98.63%           
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  Files          35       35           
  Lines        5860     5860           
=======================================
  Hits         5780     5780           
  Misses         80       80           

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[utkarshpawade]

The ubuntu-latest (devel) failure is pre-existing on stan-dev:master and is not related to this PR. It was introduced when PR #424 was merged into master today. The changes in this PR are documentation-only (NEWS.md, roxygen docs) and do not affect any plots, test logic, or snapshot files. The 5 other CI checks all pass ✅.