docs: guide on how to uninstall/purge hasura

Dear Hasura people, we're using your amazing hasura.cli-migrations docker container but as part of a pipeline, I would need to completely kill any hasura objects in my database in order to rebuild them. Having some dependency issues at the moment (specifically INSTEAD OF triggers that seem to get created when granting some partial INSERT permissions)

Is it as simple as dropping the `hdb_views` and `hdb_catalog` schemas?

I would suggest adding this information to your also-amazing documentation.

Thanks!