docs: improve schema extensibility documentation and samples

- Added TSDoc for 'createHierarchicalValueFieldResolver'.

- Updated README with 'Extending the Schema' guide and Zabbix preconditions.

- Migrated all MQTT items to Agent 2 'mqtt.get' format across documentation and test data.

- Added 'docs/sample_import_distance_tracker_template.graphql' as a schema extension example.

- Verified all 38 tests pass.

docs/sample_import_templates_mutation.graphql

@@ -56,14 +56,14 @@ This sample data is based on the `BT_DEVICE_TRACKER` template from `src/testdata
             }
           ],
           "master_item": {
-            "key": "mqtt.trap[deviceValue/location]"
+            "key": "mqtt.get[\"tcp://mqtt-broker:1883\",\"deviceValue/location\"]"
           }
         },
         {
           "uuid": "380c4a7d752848cba3b5a59a0f9b13c0",
           "name": "MQTT_LOCATION",
-          "type": 2,
-          "key": "mqtt.trap[deviceValue/location]",
+          "type": 0,
+          "key": "mqtt.get[\"tcp://mqtt-broker:1883\",\"deviceValue/location\"]",
           "value_type": 4,
           "history": "0"
         }
@@ -77,6 +77,7 @@ This sample data is based on the `BT_DEVICE_TRACKER` template from `src/testdata
 When converting from Zabbix YAML/XML exports, use the following numeric mappings for items and preprocessing:
 
 #### Item Type (`type`)
+- `0`: ZABBIX_AGENT
 - `2`: ZABBIX_TRAP (TRAP)
 - `18`: DEPENDANT_ITEM (DEPENDENT)
 - `21`: SIMULATOR_JAVASCRIPT (JAVASCRIPT)