[Docs] Add dev docs for saved object IDs

[jportner]

Many consumers of saved objects rely on the object IDs, either directly or indirectly.

With the introduction of sharing saved objects, many saved object types have been converted to become "share-capable" in the 8.0 release, causing some object IDs to change to enforce globally unique IDs for this object type (#100489).

However, object IDs can change on upgrade to 8.0 and/or when importing/copying saved objects. Changing IDs on upgrade is documented here, but changing IDs on import/copy isn't.

Further, upgrading a single old object by importing it into a newer version of Kibana (triggering a "document migration") doesn't behave exactly the same way as upgrading all old objects by installing a new version of Kibana (triggering an "index migration") -- more details here.

We should put some additional dev docs on consuming saved objects, what to expect when it comes to share-capable object IDs, and best practices for testing migrations/imports.

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)