Style Engine: add first draft of contributing doc

[ramonjd]

What?

Adding some contributor notes that highlight important aspects of Style Engine development.

This is a first draft.

Style Engine tracking issue:

• Tracking: Add a Style Engine to manage rendering block styles #38167

Why?

To explain the build process and naming conventions.

Testing Instructions

Run npm run test:unit:php -- --group style-engine to check that the @group annotation works.

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[andrewserong]

Looks great to me, thanks for adding this in! 🎉

There was one typo, and I left a few little nit comments, but feel free to ignore them if you don't think they're applicable 🙂

[ramonjd]

Thanks for the quick review/proofread @andrewserong

Suggestions committed!