Add docs for changing domain names

This comes up occasionally on the forum and I think it should be in the docs.

* If you have devices or dashboards that need to use that same old domain, [use a CNAME redirect](https://forum.getodk.org/t/migrate-central-from-one-server-to-another/48965/2).
* If you want a complete break from that domain, you can find/replace the domain by following the "Create Domain Update Script Files" section of [@KagundaJM](https://forum.getodk.org/u/kagundajm)'s [migration guide](https://dev.to/kagundajm/odk-central-domain-change-a-painless-backup-and-restore-tutorial-4gpk).

Start with the Sphinx documentation and review the linked forum answer and migration guide. Add guidance for both keeping the old domain with a CNAME redirect and completely replacing the domain using the domain update script approach. Done means users can choose the appropriate migration path from the docs.

Written by the indexing model from the issue text.