Merge branch 'jl-simplify-mattermost-docs' into 'master'

Crosslink Mattermost integrations, simplify components list

See merge request gitlab-org/gitlab-ce!29317

doc/development/architecture.md

@@ -149,7 +149,7 @@ Component statuses are linked to configuration documentation for each component.
 | [PgBouncer Exporter](#pgbouncer-exporter) | Prometheus endpoint with PgBouncer metrics | [⚙][pgbouncer-exporter-omnibus] | [❌][pgbouncer-exporter-charts] | [❌][pgbouncer-exporter-charts] | [✅](https://about.gitlab.com/handbook/engineering/monitoring/) | ❌ | ❌ | CE & EE |
 | [GitLab Monitor](#gitlab-monitor) | Generates a variety of GitLab metrics | [✅][gitlab-monitor-omnibus] | [❌][gitab-monitor-charts] | [❌][gitab-monitor-charts] | [✅](https://about.gitlab.com/handbook/engineering/monitoring/) | ❌ | ❌ | CE & EE |
 | [Node Exporter](#node-exporter) | Prometheus endpoint with system metrics | [✅][node-exporter-omnibus] | [❌][node-exporter-charts] | [❌][node-exporter-charts] | [✅](https://about.gitlab.com/handbook/engineering/monitoring/) | ❌ | ❌ | CE & EE |
-| [Mattermost](#mattermost) | Open-source Slack alternative | [⚙][mattermost-omnibus] | [⤓][mattermost-charts] | [⤓][mattermost-charts] | [⤓](../user/project/integrations/mattermost_slash_commands.md#manual-configuration), [⤓](../user/project/integrations/mattermost.html) | ❌ | ❌ | CE & EE |
+| [Mattermost](#mattermost) | Open-source Slack alternative | [⚙][mattermost-omnibus] | [⤓][mattermost-charts] | [⤓][mattermost-charts] | [⤓](../user/project/integrations/mattermost.md) | ❌ | ❌ | CE & EE |
 | [MinIO](#minio) | Object storage service | [⤓][minio-omnibus] | [✅][minio-charts] | [✅][minio-charts] | [✅](https://about.gitlab.com/handbook/engineering/infrastructure/production-architecture/#storage-architecture) | ❌ | [⚙][minio-gdk] | CE & EE |
 | [Runner](#gitlab-runner) | Executes GitLab CI jobs | [⤓][runner-omnibus] | [✅][runner-charts] | [⚙][runner-charts] | [✅](../user/gitlab_com/index.md#shared-runners) | [⚙][runner-source] | [⚙][runner-gdk] | CE & EE |
 | [Database Migrations](#database-migrations) | Database migrations | [✅][database-migrations-omnibus] | [✅][database-migrations-charts] | [✅][database-migrations-charts] | ✅ | [⚙][database-migrations-source] | ✅ | CE & EE |

doc/user/project/integrations/mattermost.md

 # Mattermost Notifications Service
 
+The Mattermost Notifications Service allows your GitLab project to send events (e.g., `issue created`) to your existing Mattermost team as notifications. This requires configurations in both Mattermost and GitLab.
+
+You can also use Mattermost slash commands to control GitLab inside Mattermost. This is the separately configured [Mattermost slash commands](mattermost_slash_commands.md).
+
 ## On Mattermost
 
 To enable Mattermost integration you must create an incoming webhook integration:

doc/user/project/integrations/mattermost_slash_commands.md

@@ -6,6 +6,9 @@ Mattermost commands give users an extra interface to perform common operations
 from the chat environment. This allows one to, for example, create an issue as
 soon as the idea was discussed in Mattermost.
 
+GitLab can also send events (e.g., `issue created`) to Mattermost as notifications.
+This is the separately configured [Mattermost Notifications Service](mattermost.md).
+
 ## Prerequisites
 
 Mattermost 3.4 and up is required.