docs: ecosystem/integrations update 5 (#5752)

- added missed integration to `docs/ecosystem/integrations/`
- updated notebooks to consistent format: changed titles, file names;
added descriptions

#### Who can review?
 @hwchase17 
 @dev2049

docs/integrations/openai.md

@@ -35,7 +35,6 @@ from langchain.llms import AzureOpenAI
 For a more detailed walkthrough of the `Azure` wrapper, see [this notebook](../modules/models/llms/integrations/azure_openai_example.ipynb)
 
 
-
 ## Text Embedding Model
 
 ```python
@@ -44,6 +43,14 @@ from langchain.embeddings import OpenAIEmbeddings
 For a more detailed walkthrough of this, see [this notebook](../modules/models/text_embedding/examples/openai.ipynb)
 
 
+## Chat Model
+
+```python
+from langchain.chat_models import ChatOpenAI
+```
+For a more detailed walkthrough of this, see [this notebook](../modules/models/chat/integrations/openai.ipynb)
+
+
 ## Tokenizer
 
 There are several places you can use the `tiktoken` tokenizer. By default, it is used to count tokens