Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2669

Removed in 04b9483f0d (2022-11-28) when switching from matrix-postgres to
the devture-postgres external Ansible role.
More details: https://github.com/spantaleev/matrix-docker-ansible-deploy/blob/master/CHANGELOG.md#matrix-postgres-has-been-replaced-by-the-comdevtureansiblerolepostgres-external-role

The `import_synapse_sqlite_db.yml` file and documentation has been adapted somewhat compared to before, so that:

- it doesn't try to start Postgres automatically. You need to handle
  this part manually
- it doesn't rely on the integrated Postgres and may potentially work
  with external Postgres instances just the same
- it doesn't wipe out the whole database anymore. By default, we assume
  it's empty anyway and there's no need for such things. If it's not,
  then it's also probably dangerous to be so destructive.

This is all completely untested, but will hopefully work.