Revert "Temporarly remove Error Tracking custom fingerprint docs for …

…RUM (#17297)"

This reverts commit 8ef6aab.

config/_default/menus/menus.en.yaml

@@ -3567,6 +3567,11 @@ main:
     parent: rum_error_tracking
     identifier: rum_error_tracking_flutter
     weight: 1407
+  - name: Custom Grouping
+    url: real_user_monitoring/error_tracking/custom_grouping
+    parent: rum_error_tracking
+    identifier: rum_error_tracking_custom_grouping
+    weight: 1408
   - name: Guides
     url: real_user_monitoring/guide/
     parent: rum

content/en/real_user_monitoring/error_tracking/custom_grouping.md

@@ -0,0 +1,62 @@
+---
+title: Custom Grouping
+kind: documentation
+description: Customize how RUM errors are grouped into issues.
+further_reading:
+- link: "https://github.com/DataDog/datadog-ci/tree/master/src/commands/sourcemaps"
+  tag: "GitHub"
+  text: "datadog-ci Source code"
+- link: "/real_user_monitoring/guide/upload-javascript-source-maps"
+  tag: "Documentation"
+  text: "Upload JavaScript source maps"
+- link: "/real_user_monitoring/error_tracking/"
+  tag: "Documentation"
+  text: "Learn about Error Tracking for Web and Mobile Applications"
+---
+
+## Overview
+
+Error Tracking intelligently groups similar errors into issues with a default strategy. By using _custom fingerprinting_, you can gain full control over the grouping decision and customize the grouping behavior for your Real User Monitoring (RUM)
+errors.
+
+Provide an `error.fingerprint` attribute that Error Tracking can use to group RUM errors into issues. While the value of the `error.fingerprint` attribute does not have any particular format or requirement, the content must be a string.
+
+If `error.fingerprint` is provided, the grouping behavior follows these rules:
+
+* Custom grouping takes precedence over the default strategy.
+* Custom grouping can be applied only to a subset of your RUM errors and can co-exist with the default strategy.
+* The content of `error.fingerprint` is used as-is without any modification.
+* RUM errors from the same service and with the same `error.fingerprint` attribute are grouped into the same issue.
+* RUM errors with different `service` attributes are grouped into different issues.
+
+## Setup
+
+Custom grouping only needs a RUM error and an `error.fingerprint` string attribute.
+
+If you aren’t already collecting RUM events with Datadog, see the [RUM documentation][1] to set up Real User Monitoring.
+
+### Example
+
+If you're already sending RUM events, add a new `error.fingerprint` attribute to your RUM error event.
+
+Here's an example for [collecting browser errors][2]:
+
+```javascript
+import { datadogRum } from '@datadog/browser-rum';
+
+// Send a custom error with context
+const error = new Error('Something went wrong');
+datadogRum.addError(error, {
+  'error.fingerprint': 'my-custom-grouping-material',
+});
+```
+
+In this case, `my-custom-grouping-material` is used to group these RUM errors into a single
+issue in Error Tracking.
+
+## Further Reading
+
+{{< partial name="whats-next/whats-next.html" >}}
+
+[1]: /real_user_monitoring/
+[2]: /real_user_monitoring/browser/collecting_browser_errors/