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/index.rst

@@ -134,25 +134,24 @@ Reference Docs
    :hidden:
 
    ./reference/installation.md
-   ./reference/integrations.md
    ./reference.rst
 
 
-LangChain Ecosystem
--------------------
+Ecosystem
+------------
 
-| Guides for how other companies/products can be used with LangChain.
+| Guides for how other products can be used with LangChain.
 
-- `LangChain Ecosystem <./ecosystem.html>`_
+- `Integrations <./integrations.html>`_
 
 .. toctree::
-   :maxdepth: 1
+   :maxdepth: 2
    :glob:
    :caption: Ecosystem
    :name: ecosystem
    :hidden:
 
-   ./ecosystem.rst
+   ./integrations.rst
 
 
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