docs: Add docs for the push source (#2461)

* docs: Add docs for the push source

Signed-off-by: Achal Shah <achals@gmail.com>

* remove bad ref

Signed-off-by: Achal Shah <achals@gmail.com>

* cr

Signed-off-by: Achal Shah <achals@gmail.com>

* cr

Signed-off-by: Achal Shah <achals@gmail.com>

docs/reference/data-sources/README.md

@@ -21,3 +21,7 @@ Please see [Data Source](../../getting-started/concepts/feature-view.md#data-sou
 {% content-ref url="../offline-stores/spark.md" %}
 [spark.md](../offline-stores/spark.md)
 {% endcontent-ref %}
+
+{% content-ref url="push.md" %}
+[push.md](push.md)
+{% endcontent-ref %}

docs/reference/data-sources/push.md

@@ -0,0 +1,42 @@
+# Push source
+
+## Description
+
+Push sources allow feature values to be pushed to the online store in real time. This allows fresh feature values to be made available to applications. Push sources supercede the 
+[FeatureStore.write_to_online_store](https://rtd.feast.dev/en/latest/index.html#feast.feature_store.FeatureStore.write_to_online_store).
+
+Push sources can be used by multiple feature views. When data is pushed to a push source, Feast propagates the feature values to all the consuming feature views.
+
+Push sources must have a batch source specified, since that's the source used when retrieving historical features. 
+When using a PushSource as a stream source in the definition of a feature view, a batch source doesn't need to be specified in the definition explicitly.
+
+## Example
+### Defining a push source
+
+```python
+from feast import PushSource, ValueType, BigQuerySource, FeatureView, Feature
+
+push_source = PushSource(
+    name="push_source",
+    schema={"user_id": ValueType.INT64, "life_time_value": ValueType.INT64},
+    batch_source=BigQuerySource(table="test.test"),
+)
+
+fv = FeatureView(
+    name="feature view",
+    entities=["user_id"],
+    features=[Feature(name="life_time_value", dtype=ValueType.INT64)],
+    stream_source=push_source,
+)
+```
+
+### Pushing data
+```python
+from feast import FeatureStore
+import pandas as pd
+
+fs = FeatureStore(...)
+feature_data_frame = pd.DataFrame()
+fs.push("push_source", feature_data_frame)
+```
+