Add examples of creating and updating entities in a single form

https://forum.getodk.org/t/updating-and-creating-entities-with-one-follow-up-form/45385/6?u=ln

Start by reviewing the linked forum discussion, especially the referenced reply, to understand the expected examples for creating and updating entities in one follow-up form. Then locate the relevant entity and follow-up form documentation in this Sphinx project. Done means the documentation includes clear, working examples covering both operations in a single form.

Written by the indexing model from the issue text.