Merge branch 'docs-remove-geo-node' into 'master'

Add doc for removing Geo node

See merge request gitlab-org/gitlab-ee!8868

doc/administration/geo/replication/index.md

@@ -235,6 +235,10 @@ For more information on Geo security, see [Geo security review](security_review.
 
 For more information on tuning Geo, see [Tuning Geo](tuning.md).
 
+## Remove Geo node
+
+For more information on removing a Geo node, see [Removing **secondary** Geo nodes](remove_geo_node.md).
+
 ## Current limitations
 
 CAUTION: **Caution:**

doc/administration/geo/replication/remove_geo_node.md

+# Removing secondary Geo nodes
+
+**Secondary** nodes can be removed from the Geo cluster using the Geo admin page of the **primary** node. To remove a **secondary** node:
+
+1. Navigate to **Admin Area > Geo** (`/admin/geo/nodes`).
+1. Click the **Remove** button for the **secondary** node you want to remove.
+1. Confirm by clicking **Remove** when the prompt appears.
+
+Once removed from the Geo admin page, you must stop and uninstall the **secondary** node:
+
+1. On the **secondary** node, stop GitLab:
+
+    ```bash
+    sudo gitlab-ctl stop
+    ```
+1. On the **secondary** node, uninstall GitLab:
+
+    ```bash
+    # Stop gitlab and remove its supervision process
+    sudo gitlab-ctl uninstall
+    
+    # Debian/Ubuntu
+    sudo dpkg --remove gitlab-ee
+    
+    # Redhat/Centos
+    sudo rpm --erase gitlab-ee
+    ```
+
+Once GitLab has been uninstalled from the **secondary** node, the replication slot must be dropped from the **primary** node's database as follows:
+
+1. On the **primary** node, start a PostgreSQL console session:
+
+    ```bash
+    sudo gitlab-psql 
+    ```
+    
+    NOTE: **Note:**
+    Using `gitlab-rails dbconsole` will not work, because managing replication slots requires superuser permissions.
+
+1. Find the name of the relevant replication slot. This is the slot that is specified with `--slot-name` when running the replicate command: `gitlab-ctl replicate-geo-database`.
+
+    ```sql
+    SELECT * FROM pg_replication_slots;
+    ```
+    
+1. Remove the replication slot for the **secondary** node:
+
+    ```sql
+    SELECT pg_drop_replication_slot('<name_of_slot>');
+    ```