Adding initial outline of pgbouncer documentation

For gitlab-org/omnibus-gitlab#2649

doc/administration/high_availability/pgbouncer.md

+# Working with the bundle Pgbouncer service
+
+## Overview
+
+As part of its High Availability stack, GitLab Premium includes a bundled version of [Pgbouncer](https://pgbouncer.github.io/) that can be managed through `/etc/gitlab/gitlab.rb`.
+
+In a High Availability setup, Pgbounce is used to seamlessly migrate database connections between servers in a failover scenario.
+
+Additionally, it can be used in a non-HA setup to pool connections, speeding up response time while reducing resource usage.
+
+It is recommended to run pgbouncer alongside the `gitlab-rails` service, or on its own dedicated node in a cluster.
+
+## Operations
+
+### Running Pgbouncer as part of an HA GitLab installation
+See our [HA documentation for PostgreSQL](database.md) for information on running pgbouncer as part of a HA setup
+
+### Running Pgbouncer as part of a non-HA GitLab installation
+
+### Interacting with pgbouncer
+
+## Troubleshooting
+
+### Debugging connection issues
+