Add ABTShield User ID docs (#6559)

* Add ABTShield User ID docs

* Update dev-docs/modules/userid-submodules/abtshield.md

Co-authored-by: Muki Seiler <muuki88@users.noreply.github.com>

* fixed lines

---------

Co-authored-by: Muki Seiler <muuki88@users.noreply.github.com>

Author: tzwierzchowski

dev-docs/modules/userid-submodules/abtshield.md

@@ -0,0 +1,78 @@
+---
+layout: userid
+title: ABTShield ID
+description: ABTShield ID User ID sub-module
+useridmodule: abtshieldIdSystem
+bidRequestUserId: abtshieldId
+eidsource: abtshield.com
+example: '"abc-123-def-456"'
+---
+
+ABTShield provides cookie-less user identification and audience segmentation for publishers. The module fetches a unique user UUID and optional audience segments from the ABTShield MCR endpoint and surfaces them to bid adapters via the standard Prebid.js `user.eids` interface.
+
+For more information and to obtain a service ID, visit [abtshield.com](https://abtshield.com).
+
+## ABTShield ID Prerequisites
+
+Before using this module you need a **service ID (`sid`)** - register at [abtshield.com](https://abtshield.com) to obtain a unique identifier for your integration. You must also whitelist every domain from which the module will be loaded in the ABTShield dashboard.
+
+## ABTShield ID Configuration
+
+Add the module to your Prebid.js build:
+
+```bash
+gulp build --modules=userId,abtshieldIdSystem
+```
+
+Then configure the module in your `userSync` configuration:
+
+```javascript
+pbjs.setConfig({
+    userSync: {
+        userIds: [{
+            name: 'abtshieldId',
+            params: {
+                sid: 'pb.your-service-id'  // required
+            },
+            storage: {
+                type: 'html5',
+                name: 'abtshield_id',
+                expires: 1
+            }
+        }]
+    }
+});
+```
+
+{: .table .table-bordered .table-striped }
+| Param | Scope | Type | Description |
+| --- | --- | --- | --- |
+| `params.sid` | Required | String | Service ID obtained from abtshield.com. |
+| `params.endpoint` | Optional | String | Override the MCR endpoint. For test environments only. Must use HTTPS. |
+| `storage` | Recommended | Object | Standard Prebid User ID storage block (`html5` or `cookie`). |
+
+## ABTShield ID Example
+
+This will add a `userId.abtshieldId` property to bid requests:
+
+```javascript
+{
+  abtshieldId: {
+    uuid: 'abc-123-def-456',
+    segments: ['seg-1', 'seg-2']  // omitted when no segments are returned
+  }
+}
+```
+
+The `user.eids` entry will look like:
+
+```javascript
+{
+  source: 'abtshield.com',
+  uids: [{
+    id: 'abc-123-def-456',
+    atype: 1,
+    ext: { segments: ['seg-1', 'seg-2'] }  // omitted when no segments are returned
+  }]
+}
+```