Update Central docs for prebuilt images

[matthew-white]

Related:

• Prebuild Frontend central#656
• Use images for all services central#677
• Use pre-built images for nginx & service containers central#895

Checklist:

• Update deployment instructions
• In the docs here, 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."