Update alpha database documentation to reflect the current state

* Change layout to clarify differenet stages of the install
* Add instructions for pgbouncer and gitlab-rails
* Remove requirement for database user password.
* repmgrd is automatically setup now, so remove manual steps

doc/administration/external_database.md

+## Configure GitLab using an external PostgreSQL service
+If you're hosting GitLab on a cloud provider, you can optionally use a
+managed service for PostgreSQL. For example, AWS offers a managed Relational
+Database Service (RDS) that runs PostgreSQL.
+
+Alternatively, you may opt to manage your own PostgreSQL instance or cluster
+separate from the GitLab Omnibus package.
+
+If you use a cloud-managed service, or provide your own PostgreSQL instance:
+
+1. Setup PostgreSQL according to the
+   [database requirements document](../install/requirements.md#database).
+1. Set up a `gitlab` username with a password of your choice. The `gitlab` user
+   needs privileges to create the `gitlabhq_production` database.
+1. Configure the GitLab application servers with the appropriate details.
+   This step is covered in [Configuring GitLab for HA](gitlab.md).