Document `propagate_correlation_id` configuration for GitLab Pages

Related to https://gitlab.com/gitlab-org/gitlab-pages/-/merge_requests/438

changelogs/unreleased/document-propagate-correlation-id-for-pages.yml

+---
+title: Document propagate_correlation_id configuration for GitLab Pages
+merge_request: 55205
+author: Ercan Ucan @ercan.ucan
+type: added

doc/administration/pages/index.md

@@ -247,6 +247,7 @@ control over how the Pages daemon runs and serves content in your environment.
 | `log_directory`                         | Absolute path to a log directory. |
 | `log_format`                            | The log output format: `text` or `json`. |
 | `log_verbose`                           | Verbose logging, true/false. |
+| `propagate_correlation_id`              | Set to true (false by default) to re-use existing Correlation ID from the incoming request header `X-Request-ID` if present. If a reverse proxy sets this header, the value will be propagated in the request chain. |
 | `max_connections`                       | Limit on the number of concurrent connections to the HTTP, HTTPS or proxy listeners. |
 | `metrics_address`                       | The address to listen on for metrics requests. |
 | `redirect_http`                         | Redirect pages from HTTP to HTTPS, true/false. |
@@ -526,6 +527,25 @@ Follow the steps below to configure verbose logging of GitLab Pages daemon.
 
 1. [Reconfigure GitLab](../restart_gitlab.md#omnibus-gitlab-reconfigure).
 
+## Propagating the correlation ID
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab-pages/-/merge_requests/438) in GitLab 13.10.
+
+Setting the `propagate_correlation_id` to true will allow installations behind a reverse proxy generate
+and set a correlation ID to requests sent to GitLab Pages. When a reverse proxy sets the header value `X-Request-ID`,
+the value will be propagated in the request chain.
+Users [can find the correlation ID in the logs](../troubleshooting/tracing_correlation_id.md#identify-the-correlation-id-for-a-request).
+
+To enable the correlation ID:
+
+1. Set the parameter to true in `/etc/gitlab/gitlab.rb`:
+
+   ```ruby
+   gitlab_pages['propagate_correlation_id'] = true
+   ```
+
+1. [Reconfigure GitLab](../restart_gitlab.md#omnibus-gitlab-reconfigure).
+
 ## Change storage path
 
 Follow the steps below to change the default path where GitLab Pages' contents