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/integrations/arxiv.md

@@ -0,0 +1,28 @@
+# Arxiv
+
+>[arXiv](https://arxiv.org/) is an open-access archive for 2 million scholarly articles in the fields of physics, 
+> mathematics, computer science, quantitative biology, quantitative finance, statistics, electrical engineering and 
+> systems science, and economics.
+
+
+## Installation and Setup
+
+First, you need to install `arxiv` python package.
+
+```bash
+pip install arxiv
+```
+
+Second, you need to install `PyMuPDF` python package which transforms PDF files downloaded from the `arxiv.org` site into the text format.
+
+```bash
+pip install pymupdf
+```
+
+## Document Loader
+
+See a [usage example](../modules/indexes/document_loaders/examples/arxiv.ipynb).
+
+```python
+from langchain.document_loaders import ArxivLoader
+```