Doc: Rework Docker documentation

[sgillespie]

Description

Updates the docker documentation to (hopefully!) be more organized and up-to-date. Here is the rendered version.

I've also created a new docker-compose.example.yml. The ideas around this are:

1. Discourage usage in non-dev workflows
2. Encourage users to maintain their own version
3. Avoid tying this file to db-sync releases

To that end, I've left the existing one for now with the idea that we'll remove it in a few months.

Checklist

• [x] Commit sequence broadly makes sense
• [x] Commits have useful messages
• [ ] New tests are added if needed and existing tests are updated
• [ ] Any changes are noted in the changelog
• [ ] Code is formatted with fourmolu on version 0.10.1.0 (which can be run with scripts/fourmolize.sh)
• [x] Self-reviewed the diff

Migrations

• [ ] The pr causes a breaking change of type a,b or c
• [ ] If there is a breaking change, the pr includes a database migration and/or a fix process for old values, so that upgrade is possible
• [ ] Resyncing and running the migrations provided will result in the same database semantically

If there is a breaking change, especially a big one, please add a justification here. Please elaborate more what the migration achieves, what it cannot achieve or why a migration is not possible.