big docs refactor (#1978)

Co-authored-by: Ankush Gola <ankush.gola@gmail.com>

docs/modules/models.rst

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+Models
+==========================
+
+.. note::
+   `Conceptual Guide <https://docs.langchain.com/docs/components/models>`_
+
+
+This section of the documentation deals with different types of models that are used in LangChain.
+On this page we will go over the model types at a high level,
+but we have individual pages for each model type.
+The pages contain more detailed "how-to" guides for working with that model,
+as well as a list of different model providers.
+
+**LLMs**
+
+Large Language Models (LLMs) are the first type of models we cover.
+These models take a text string as input, and return a text string as output.
+
+
+**Chat Models**
+
+Chat Models are the second type of models we cover.
+These models are usually backed by a language model, but their APIs are more structured.
+Specifically, these models take a list of Chat Messages as input, and return a Chat Message.
+
+**Text Embedding Models**
+
+The third type of models we cover are text embedding models.
+These models take text as input and return a list of floats.
+
+
+Go Deeper
+---------
+
+.. toctree::
+   :maxdepth: 1
+
+   ./models/llms.rst
+   ./models/chat.rst
+   ./models/text_embedding.rst