Update Central docs for prebuilt images

Related:

- getodk/central#656
- getodk/central#677
- getodk/central#895

Checklist:

- [ ] Update deployment instructions
- [ ] In the docs [here](https://docs.getodk.org/central-install-digital-ocean/#using-a-custom-ssl-certificate), we suggest rebuilding `nginx`. But @lognaturel says that with v2024.2, `nginx` will just need to be restarted: "The logic to put the certs in the right place and everything is in the command that’s run at container start time."
- [ ] @lognaturel: "In general I think we’ll have to edit docs to say build before v2024.2 [or whichever release it is] and stop and restart in v2024.2 and after."

Start with the Central deployment instructions and the custom SSL certificate page linked in the issue. Review related issues getodk/central#656, #677, and #895 to confirm the release boundary, then update the instructions for versions before v2024.2 and v2024.2 or later. Done means the checklist is addressed and the documented nginx operation matches the prebuilt-image behavior.

Written by the indexing model from the issue text.