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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