Add Gemius ID submodule documentation. (#6191)

Author: mmuras

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

@@ -0,0 +1,44 @@
+---
+layout: userid
+title: Gemius ID
+description: Gemius ID User ID sub-module
+useridmodule: gemiusIdSystem
+bidRequestUserId: gemiusId
+eidsource: gemius.com
+example: '"1111"'
+---
+
+The Gemius ID submodule enables publishers to access the RUID identifier provided by gemius.com
+within the Prebid.js ecosystem.
+
+Add support for Gemius ID to your Prebid.js package using:
+
+```bash
+gulp build --modules=userId,gemiusIdSystem
+```
+
+## Gemius ID Registration
+
+This submodule requires a script provided by gemius.com. For more information, please contact us at: [https://gemius.com/contact/](https://gemius.com/contact/).
+
+## Gemius ID Configuration
+
+The Gemius ID module does not require any configuration parameters except `params.storage` as provided in example
+
+## Gemius ID Example
+
+```javascript
+pbjs.setConfig({
+    userSync: {
+        userIds: [{
+            name: 'gemiusId',
+            storage: {
+                name: 'pbjs_gemiusId',
+                type: 'cookie',
+                expires: 30,
+                refreshInSeconds: 3600
+            }
+        }]
+    }
+});
+```