Postgres upgrade instructions missing

Postgres has recently (in 9469c231add3f98c262bc51a088f147571501f36) been upgraded from 9.6 to 13.4 in the default/example `docker-compose.yml` file.

Users just upgrading the container repo will face the following error:

The data directory was initialized by PostgreSQL version 9.6, which is not compatible with this version 13.4.

Database upgrade instructions should be added to the [docker upgrade notes](https://docs.hedgedoc.org/setup/docker/).