Add docs links to built-in implementations

Author: deliahu

docs/applications/implementations/aggregators.md

@@ -22,6 +22,8 @@ def aggregate_spark(data, input):
     pass
 ```
 
+See Cortex's built-in <!-- CORTEX_VERSION_MINOR -->[`aggregators`](https://github.com/cortexlabs/cortex/blob/master/pkg/aggregators)) for example implementations.
+
 ## Example
 
 ```python

docs/applications/implementations/estimators.md

@@ -28,6 +28,8 @@ def create_estimator(run_config, model_config):
 
 See the [tf.estimator.RunConfig](https://www.tensorflow.org/api_docs/python/tf/estimator/RunConfig) and [tf.estimator.Estimator](https://www.tensorflow.org/api_docs/python/tf/estimator/Estimator) documentation for more information.
 
+See Cortex's built-in <!-- CORTEX_VERSION_MINOR -->[`estimators`](https://github.com/cortexlabs/cortex/blob/master/pkg/estimators)) for example implementations.
+
 ## Example
 
 ```python

docs/applications/implementations/transformers.md

@@ -65,6 +65,8 @@ def reverse_transform_python(transformed_value, input):
     pass
 ```
 
+See Cortex's built-in <!-- CORTEX_VERSION_MINOR -->[`transformers`](https://github.com/cortexlabs/cortex/blob/master/pkg/transformers)) for example implementations.
+
 ## Example
 
 ```python