Create developer documentation for modifying the schema

[mferrera]

We are collecting some learnings about the nuances of the mapping from Pydantic models to a produced schema. We should document these somewhere to help both future developers understand them, and also for reviewers to better understand possible implications of incoming changes.

Some important things to highlight:

• The ordering and matching of unioned types (MAINT: Fix pydantic serialization warning #824)
• The content of Consider disallowing fields to be nullable #813
• RootModels

For some of these it may be good idea to collect them as part of a PR checklist.