DM-48178: Add documentation for DRP schemas.

[TallJimbo]

Checklist

When making changes to YAML files in the schemas directory:

• If applicable, incremented the schema version number, following the guidelines in the contribution guide
• Referred to the documentation on specific schemas for additional versioning information, change constraints, or tasks that may need to be performed, based on which schema is being updated

[JeremyMcCormick]

Thank you for this contribution, @TallJimbo !

This should be useful information, especially for repository contributors.

I have made a few relatively minor inlined suggestions:

• Added links for referenced GitHub packages and files so that they are more easily accessible.
• Changed some repository and file links to absolute style, as github.com should be unneeded.
• Made a few minor changes to the wording and grammar.

I have left this in "Request changes" state until I am able to discuss with @gpdf, in case he would like to see more major changes in the form of additional information, etc.

My only suggestion for additional text might be including some information on who should be considered responsible for reviewing PRs on the DRP schemas. You had mentioned to me that the reviewers of the pipeline updates should also be reviewing the corresponding sdm_schemas updates. Would this information be worth stating explicitly here, especially for the benefit of new Rubin staff members working on the pipelines who may not be completely familiar with the procedure?

[JeremyMcCormick]

I have made some minor edits to the original draft, mostly just adding links.

@TallJimbo This looks good to me now.

Please feel free to make minor corrections/additions before merging.