Add MongoDB configuration docs (#1796)

**What this PR does / why we need it**:

**Which issue(s) this PR fixes**:

Fixes #

**Does this PR introduce a user-facing change?**:
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This PR was merged by Kapetanios.

docs/content/en/docs/operator-manual/control-plane/configuration-reference.md

@@ -58,6 +58,18 @@ Must be one of the following objects:
 | passwordFile | string | Path to the file containing the password. | No |
 
 
+### DataStoreMongoDBConfig
+
+> Note: `deprecated` feature (please use `Firestore` or `MySQL` instead)
+
+| Field | Type | Description | Required |
+|-|-|-|-|
+| url | string | The address to MongoDB server. | Yes |
+| database | string | The name of database. | Yes |
+| usernameFile | string | The path to the username file. | No |
+| passwordFile | string | The path to the password file. | No |
+
+
 ## FileStore
 
 | Field | Type | Description | Required |