Improve section on changing form structure

Some good ideas from this conversation: https://forum.getodk.org/t/backend-consequences-of-modifying-odk-collect-forms/56772
* Explicitly say that renaming a field is like deleting it and adding a new one
* Be explicit about changing group/repeat structure

Could be structured as a FAQ?

Start with the linked forum conversation to understand the backend consequences of changing ODK Collect forms, then locate the existing documentation section covering form changes. Done means clearly explaining field renames and group/repeat structure changes, potentially in an FAQ format.

Written by the indexing model from the issue text.