docs: ecosystem/integrations update 1 (#5219)

# docs: ecosystem/integrations update

It is the first in a series of `ecosystem/integrations` updates.

The ecosystem/integrations list is missing many integrations.
I'm adding the missing integrations in a consistent format: 
1. description of the integrated system
2. `Installation and Setup` section with 'pip install ...`, Key setup,
and other necessary settings
3. Sections like `LLM`, `Text Embedding Models`, `Chat Models`... with
links to correspondent examples and imports of the used classes.

This PR keeps new docs, that are presented in the
`docs/modules/models/text_embedding/examples` but missed in the
`ecosystem/integrations`. The next PRs will cover the next example
sections.

Also updated `integrations.rst`: added the `Dependencies` section with a
link to the packages used in LangChain.

## Who can review?

@hwchase17
@eyurtsev
@dev2049

docs/modules/indexes/document_loaders/examples/arxiv.ipynb

@@ -47,7 +47,7 @@
     "tags": []
    },
    "source": [
-    "Second, you need to install `PyMuPDF` python package which transform PDF files from the `arxiv.org` site into the text format."
+    "Second, you need to install `PyMuPDF` python package which transforms PDF files downloaded from the `arxiv.org` site into the text format."
    ]
   },
   {