docs ecosystem/integrations update 3 (#5470)

# docs: `ecosystem_integrations` update 3

Next cycle of updating the `ecosystem/integrations`
* Added an integration `template` file
* Added missed integration files
* Fixed several document_loaders/notebooks

## Who can review?

Is it possible to assign somebody to review PRs on docs? Thanks.

docs/integrations/notion.md

@@ -0,0 +1,27 @@
+# Notion DB
+
+>[Notion](https://www.notion.so/) is a collaboration platform with modified Markdown support that integrates kanban 
+> boards, tasks, wikis and databases. It is an all-in-one workspace for notetaking, knowledge and data management, 
+> and project and task management.
+
+## Installation and Setup
+
+All instructions are in examples below.
+
+## Document Loader
+
+We have two different loaders: `NotionDirectoryLoader` and `NotionDBLoader`.
+
+See a [usage example for the NotionDirectoryLoader](../modules/indexes/document_loaders/examples/notion.ipynb).
+
+
+```python
+from langchain.document_loaders import NotionDirectoryLoader
+```
+
+See a [usage example for the NotionDBLoader](../modules/indexes/document_loaders/examples/notiondb.ipynb).
+
+
+```python
+from langchain.document_loaders import NotionDBLoader
+```