Add Outline provider doc (#13938)

- **Description:** Added a provider doc to `docs/integrations/providers`
for the new Outline integration in #13889
  - **Tag maintainer:** @baskaryan

docs/docs/integrations/providers/outline.mdx

@@ -0,0 +1,22 @@
+# Outline
+
+> [Outline](https://www.getoutline.com/) is an open-source collaborative knowledge base platform designed for team information sharing.
+
+## Setup
+
+You first need to [create an api key](https://www.getoutline.com/developers#section/Authentication) for your Outline instance. Then you need to set the following environment variables:
+
+```python
+import os
+
+os.environ["OUTLINE_API_KEY"] = "xxx"
+os.environ["OUTLINE_INSTANCE_URL"] = "https://app.getoutline.com"
+```
+
+## Retriever
+
+See a [usage example](/docs/integrations/retrievers/outline).
+
+```python
+from langchain.retrievers import OutlineRetriever
+```