Provide quick install steps

From https://www.reddit.com/r/selfhosted/comments/13j37tx/comment/jkeh6cl/

> One bit of advice, and only since you posted here: you mention self-hosting and open source a few places, and I have now made multiple clicks to try to find a docker compose file, but no matter what I'm given a partial set up steps then told to look at the Digital Ocean guide. You might want to just have a readily available docker compose file and .env template in your docs for people to easily copy and paste

I agree that it'd be nice to have a place that lists the steps you need to get started without so much prose. https://docs.getodk.org/central-upgrade/ does a good job of this...

Start with the linked central-upgrade documentation and the existing Digital Ocean setup guidance to find the current installation steps and deployment references. Add a concise quick-start section with a readily copyable Docker Compose file and .env template, and confirm that the resulting steps cover a fresh setup without requiring the longer guide.

Written by the indexing model from the issue text.