docs: explain parley engine shaping pipeline

[Soundcreates]

LLM Contributions: Documentation drafting and review.

This adds a concise usage overview to parley_engine's crate documentation. It explains the expected Analysis → itemization → shaping order and when higher-level users should use parley::LayoutContext.

Changelog: None

[DJMcNab]

I don't quite have enough context paged in to review this currently. However, I'd just like to understand your workflow here - we have a newly introduced PR template, which you haven't followed. What tool did you use the open this PR?

[Soundcreates]

Thanks for pointing this out. I missed the newly introduced PR template. I’ve updated the description to follow the current template and included the required disclosure.

[tomcur]

Thank you! There have been some recent large API changes, so I've folded this into #742.