docs: normalize headings to sentence case

[cnaples79]

Summary

• apply sentence case to doc titles/headings for consistency with chosen style
• replace tutorial phrase headings with concise, descriptive titles

Rationale

• aligns documentation headings with agreed sentence-case style and removes informal phrase titles (issue Inconsistent title case in docs #3819)

Changes

• updated tutorial titles and index entries to descriptive sentence case
• adjusted topic/reference/about headings (data sources, debugging, pack engine, image plugins, success stories, platforms) to sentence case

Test Plan

• not run locally; docs-only change

Fixes #3819

[HalfWhitt]

Hi, thanks for the contribution! This is currently failing the towncrier check; you'll need to add a change note.

[mhsmith]

The sidebar captions in SUMMARY.md should be updated to match the new titles. And there are a few more titles in there that you missed, such as "Release History" and "API Reference".

[HalfWhitt]

What mhsmith said, plus your changes to Pack and Image plugins — which I recommended not changing — are probably going to cause merge conflicts with #4022. I'd also recommend making the tutorial tutorial titles gerunds (with "-ing", e.g. "Organizing layouts").

Do you anticipate having the opportunity to finish work on this?