docs: compound ecosystem and integrations (#4870)

# Docs: compound ecosystem and integrations

**Problem statement:** We have a big overlap between the
References/Integrations and Ecosystem/LongChain Ecosystem pages. It
confuses users. It creates a situation when new integration is added
only on one of these pages, which creates even more confusion.
- removed References/Integrations page (but move all its information
into the individual integration pages - in the next PR).
- renamed Ecosystem/LongChain Ecosystem into Integrations/Integrations.
I like the Ecosystem term. It is more generic and semantically richer
than the Integration term. But it mentally overloads users. The
`integration` term is more concrete.
UPDATE: after discussion, the Ecosystem is the term.
Ecosystem/Integrations is the page (in place of Ecosystem/LongChain
Ecosystem).

As a result, a user gets a single place to start with the individual
integration.

docs/integrations.rst

@@ -0,0 +1,33 @@
+Integrations
+===================
+
+LangChain integrates with many LLMs, systems, and products.
+
+Integrations by Module
+--------------------------------
+
+Integrations grouped by the core LangChain module they map to:
+
+
+- `LLM Providers <./modules/models/llms/integrations.html>`_
+- `Chat Model Providers <./modules/models/chat/integrations.html>`_
+- `Text Embedding Model Providers <./modules/models/text_embedding.html>`_
+- `Document Loader Integrations <./modules/indexes/document_loaders.html>`_
+- `Text Splitter Integrations <./modules/indexes/text_splitters.html>`_
+- `Vectorstore Providers <./modules/indexes/vectorstores.html>`_
+- `Retriever Providers <./modules/indexes/retrievers.html>`_
+- `Tool Providers <./modules/agents/tools.html>`_
+- `Toolkit Integrations <./modules/agents/toolkits.html>`_
+
+
+All Integrations
+-------------------------------------------
+
+A comprehensive list of LLMs, systems, and products integrated with LangChain:
+
+
+.. toctree::
+   :maxdepth: 1
+   :glob:
+
+   integrations/*