fix: api external url should be for gotrue, not appflowy cloud

[khorshuheng]

API_EXTERNAL_URL is meant for the external endpoint for gotrue, not appflowy cloud. Not having gotrue suffix will cause SAML integration to fail.

[speed2exe]

If we want to remove gotrue prefix, we need also to modify nginx routing so that every supported path in gotrue service is routed properly. This will also break the dev env (AppFlowyCloud Develop) in appflowy flutter app if we also apply the changes to docker-compose-dev.yml.
In addition, documentations in doc/AUTHENTICATION.md will also need to be updated.

There might also be other issues, these are the concerns I can think of at the moment.

EDIT: did not see this line API_EXTERNAL_URL=http://your-host/gotrue, looks good to me