Document how to use Azure Blob storage for backups

Configuring backups is similar to but different from object storage
administration. Provide an example on how to configure backups for Azure
for completeness.

doc/raketasks/backup_restore.md

@@ -534,6 +534,44 @@ For installations from source:
 
 1. [Restart GitLab](../administration/restart_gitlab.md#installations-from-source) for the changes to take effect
 
+##### Using Azure Blob storage
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/25877) in GitLab 13.4.
+
+For Omnibus GitLab packages:
+
+1. Edit `/etc/gitlab/gitlab.rb`:
+
+   ```ruby
+   gitlab_rails['backup_upload_connection'] = {
+    'provider' => 'AzureRM',
+    'azure_storage_account_name' => '<AZURE STORAGE ACCOUNT NAME>',
+    'azure_storage_access_key' => '<AZURE STORAGE ACCESS KEY>',
+    'azure_storage_domain' => 'blob.core.windows.net', # Optional
+   }
+   gitlab_rails['backup_upload_remote_directory'] = '<AZURE BLOB CONTAINER>'
+   ```
+
+1. [Reconfigure GitLab](../administration/restart_gitlab.md#omnibus-gitlab-reconfigure) for the changes to take effect
+
+For installations from source:
+
+1. Edit `home/git/gitlab/config/gitlab.yml`:
+
+   ```yaml
+     backup:
+       upload:
+         connection:
+           provider: 'AzureRM'
+           azure_storage_account_name: '<AZURE STORAGE ACCOUNT NAME>'
+           azure_storage_access_key: '<AZURE STORAGE ACCESS KEY>'
+         remote_directory: '<AZURE BLOB CONTAINER>'
+   ```
+
+1. [Restart GitLab](../administration/restart_gitlab.md#installations-from-source) for the changes to take effect
+
+See [the table of Azure parameters](../administration/object_storage.md#azure-blob-storage) for more details.
+
 ##### Specifying a custom directory for backups
 
 Note: This option only works for remote storage. If you want to group your backups