chore: document how to use the database (#149)

## Description

This adds a configuration.md for GitLab

## Related Issue

Relates to #124

## Type of change

- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [X] Other (security config, docs update, etc)

## Checklist before merging

- [X] Test, docs, adr added or updated as needed
- [X] [Contributor Guide
Steps](https://github.com/defenseunicorns/uds-package-gitlab/blob/main/CONTRIBUTING.md#developer-workflow)
followed

docs/configuration.md

@@ -0,0 +1,43 @@
+# Configuration
+
+GitLab in this package is configured through the upstream [GitLab chart](https://docs.gitlab.com/charts/) as well as a UDS configuration chart that supports the following:
+
+## Networking
+
+Network policies are controlled via the `uds-gitlab-config` chart in accordance with the [common patterns for networking within UDS Software Factory](https://github.com/defenseunicorns/uds-software-factory/blob/main/docs/networking.md).  GitLab interacts with GitLab runners, object storage, Redis and Postgresql externally and supports the following keys:
+
+- `storage`: sets network policies for accessing object storage from all GitLab services (`registry`, `pages`, `webservice`, `toolbox`, `sidekiq`)
+- `redis`:  sets network policies for accessing a Redis-compatible server from all GitLab services (`webservice`, `toolbox`, `sidekiq`, `migrations`, `gitlab-exporter`)
+- `postgres`: sets network policies for accessing a Postgres database from all GitLab services (`webservice`, `toolbox`, `sidekiq`, `migrations`, `gitlab-exporter`)
+- `custom`: sets custom network policies for the GitLab namespace - this allows for custom integrations with other services (i.e. Jira)
+
+## Database
+
+GitLab uses Postgres as its backing database service and supports the [common database providers within UDS Software Factory](https://github.com/defenseunicorns/uds-software-factory/blob/main/docs/database.md).  
+
+### Manual Database Connection
+
+If you are using the UDS Postgres Operator or another external database that uses usernames/passwords you can use the following Helm overrides to configure it:
+
+#### `uds-gitlab-config` chart:
+
+> [!IMPORTANT]
+> The `postgres.password` setting is not applicable when using the UDS Postgres Operator package or when supplying a secret manually.
+
+- `postgres.password` - provides a password to generate a secret to pass to GitLab
+
+
+#### `gitlab` chart:
+
+> [!IMPORTANT]
+> The `global.psql.password.secret` setting is not applicable when providing a password to the `uds-gitlab-config` chart manually.
+
+- `global.psql.username` - provides the username to use when connecting to the database (i.e. `gitlab.gitlab`)
+- `global.psql.password.secret` - provides the secret that contains the database password (i.e. `gitlab.gitlab.pg-cluster.credentials.postgresql.acid.zalan.do`)
+- `global.psql.host` - provides the endpoint to use to connect to the database (i.e. `pg-cluster.postgres.svc.cluster.local`)
+
+### IAM Roles for Service Accounts
+
+The Software Factory team has not yet tested IRSA with AWS RDS - there is an open issue linked below with further linked issues to test this that could act as a starting point to implement:
+
+https://github.com/defenseunicorns/uds-software-factory/issues/45