docs: added community modules descriptions (#17827)

API Reference: Several `community` modules (like
[adapter](https://api.python.langchain.com/en/latest/community_api_reference.html#module-langchain_community.adapters)
module) are missing descriptions. It happens when langchain was split to
the core, langchain and community packages.
- Copied module descriptions from other packages
- Fixed several descriptions to the consistent format.

libs/community/langchain_community/adapters/__init__.py

@@ -0,0 +1,8 @@
+"""**Adapters** are used to adapt LangChain models to other APIs.
+
+LangChain integrates with many model providers.
+While LangChain has its own message and model APIs,
+LangChain has also made it as easy as
+possible to explore other models by exposing an **adapter** to adapt LangChain
+models to the other APIs, as to the OpenAI API.
+"""