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/gitbook.md

@@ -0,0 +1,15 @@
+# GitBook
+
+>[GitBook](https://docs.gitbook.com/) is a modern documentation platform where teams can document everything from products to internal knowledge bases and APIs.
+
+## Installation and Setup
+
+There isn't any special setup for it.
+
+## Document Loader
+
+See a [usage example](../modules/indexes/document_loaders/examples/gitbook.ipynb).
+
+```python
+from langchain.document_loaders import GitbookLoader
+```