Add Docs for Password hiding

docs/engineering/android-app/configuring/config-types/forms/questionnaire.mdx

@@ -326,3 +326,69 @@ eventType |The intention of the eventWorkflow. E.g close resources | yes | RESOU
 triggerConditions | This defines an array of condition for to be met for the event to run | no | null |
 eventResourceId |  uniqueId of resource id to be closed |  yes   |               |
 eventResources | A list of resources to close(Type of ResourceConfig) |    yes           |     |      |
+
+
+## Hiding characters in a questionnaire
+We can hide sensitive information typed on a questionnaire through adding a linkId extension. A sample linkId with password-widget extension looks like
+``` json
+{
+  "extension": [
+    {
+      "url": "https://github.com/google/android-fhir/StructureDefinition/questionnaire-itemControl",
+      "valueString": "password-widget"
+    },
+    {
+      "url": "http://hl7.org/fhir/StructureDefinition/minLength",
+      "valueInteger": 16
+    },
+    {
+      "url": "http://ehelse.no/fhir/StructureDefinition/validationtext",
+      "valueString": "NIK number should be of 16 digits"
+    },
+    {
+      "url": "http://hl7.org/fhir/StructureDefinition/regex",
+      "valueString": "^\\+?(?:[()\\h-]*\\d[()\\h-]*){16}$"
+    },
+    {
+      "url": "http://hl7.org/fhir/StructureDefinition/questionnaire-displayCategory",
+      "valueCodeableConcept": {
+        "coding": [
+          {
+            "system": "http://hl7.org/fhir/questionnaire-display-category",
+            "code": "instructions"
+          }
+        ]
+      }
+    }
+  ],
+  "linkId": "82a80049-8d2d-4008-81aa-55356b9d6628",
+  "text": "NIK number",
+  "_text": {
+    "extension": [
+      {
+        "url": "http://hl7.org/fhir/StructureDefinition/translation",
+        "extension": [
+          {
+            "url": "lang",
+            "valueCode": "id"
+          },
+          {
+            "url": "content",
+            "valueString": "Nomor Induk Kependudukan (NIK)"
+          }
+        ]
+      }
+    ]
+  },
+  "type": "string",
+  "required": false,
+  "maxLength": 16
+}
+```
+Below is the specific extension for this. The extension is validated at class called [PasswordViewHolderFactory](https://github.com/opensrp/fhircore/blob/main/android/quest/src/main/java/org/smartregister/fhircore/quest/ui/sdc/PasswordViewHolderFactory.kt)
+``` json
+  {
+          "url": "https://github.com/google/android-fhir/StructureDefinition/questionnaire-itemControl",
+          "valueString": "password-widget"
+        }
+```