docs: document_loaders classification (#4069)

**Problem statement:** the
[document_loaders](https://python.langchain.com/en/latest/modules/indexes/document_loaders.html#)
section is too long and hard to comprehend.
**Proposal:** group document_loaders by 3 classes: (see `Files changed`
tab)

UPDATE: I've completely reworked the document_loader classification.
Now this PR changes only one file! 

FYI @eyurtsev @hwchase17

docs/modules/indexes/document_loaders.rst

@@ -6,19 +6,126 @@ Document Loaders
 
 
 Combining language models with your own text data is a powerful way to differentiate them.
-The first step in doing this is to load the data into "documents" - a fancy way of say some pieces of text.
-This module is aimed at making this easy.
+The first step in doing this is to load the data into "Documents" - a fancy way of say some pieces of text.
+The document loader is aimed at making this easy.
 
-A primary driver of a lot of this is the `Unstructured <https://github.com/Unstructured-IO/unstructured>`_ python package.
-This package is a great way to transform all types of files - text, powerpoint, images, html, pdf, etc - into text data.
+
+The following document loaders are provided:
+
+
+Transform loaders
+------------------------------
+
+These **transform** loaders transform data from a specific format into the Document format.
+For example, there are **transformers** for CSV and SQL.
+Mostly, these loaders input data from files but sometime from URLs.
+
+A primary driver of a lot of these transformers is the `Unstructured <https://github.com/Unstructured-IO/unstructured>`_ python package.
+This package transforms many types of files - text, powerpoint, images, html, pdf, etc - into text data.
 
 For detailed instructions on how to get set up with Unstructured, see installation guidelines `here <https://github.com/Unstructured-IO/unstructured#coffee-getting-started>`_.
 
-The following document loaders are provided:
+
+.. toctree::
+   :maxdepth: 1
+   :glob:
+
+   ./document_loaders/examples/conll-u.ipynb
+   ./document_loaders/examples/copypaste.ipynb
+   ./document_loaders/examples/csv.ipynb
+   ./document_loaders/examples/email.ipynb
+   ./document_loaders/examples/epub.ipynb
+   ./document_loaders/examples/evernote.ipynb
+   ./document_loaders/examples/facebook_chat.ipynb
+   ./document_loaders/examples/file_directory.ipynb
+   ./document_loaders/examples/html.ipynb
+   ./document_loaders/examples/image.ipynb
+   ./document_loaders/examples/jupyter_notebook.ipynb
+   ./document_loaders/examples/markdown.ipynb
+   ./document_loaders/examples/microsoft_powerpoint.ipynb
+   ./document_loaders/examples/microsoft_word.ipynb
+   ./document_loaders/examples/pandas_dataframe.ipynb
+   ./document_loaders/examples/pdf.ipynb
+   ./document_loaders/examples/sitemap.ipynb
+   ./document_loaders/examples/subtitle.ipynb
+   ./document_loaders/examples/telegram.ipynb
+   ./document_loaders/examples/toml.ipynb
+   ./document_loaders/examples/unstructured_file.ipynb
+   ./document_loaders/examples/url.ipynb
+   ./document_loaders/examples/web_base.ipynb
+   ./document_loaders/examples/whatsapp_chat.ipynb
+
+
+
+Public dataset or service loaders
+----------------------------------
+These datasets and sources are created for public domain and we use queries to search there
+and download necessary documents.
+For example, **Hacker News** service.
+
+We don't need any access permissions to these datasets and services.
+
+
+.. toctree::
+   :maxdepth: 1
+   :glob:
+
+   ./document_loaders/examples/arxiv.ipynb
+   ./document_loaders/examples/azlyrics.ipynb
+   ./document_loaders/examples/bilibili.ipynb
+   ./document_loaders/examples/college_confidential.ipynb
+   ./document_loaders/examples/gutenberg.ipynb
+   ./document_loaders/examples/hacker_news.ipynb
+   ./document_loaders/examples/hugging_face_dataset.ipynb
+   ./document_loaders/examples/ifixit.ipynb
+   ./document_loaders/examples/imsdb.ipynb
+   ./document_loaders/examples/mediawikidump.ipynb
+   ./document_loaders/examples/youtube_transcript.ipynb
+
+
+Proprietary dataset or service loaders
+------------------------------
+These datasets and services are not from the public domain.
+These loaders mostly transform data from specific formats of applications or cloud services,
+for example **Google Drive**.
+
+We need access tokens and sometime other parameters to get access to these datasets and services.
 
 
 .. toctree::
    :maxdepth: 1
    :glob:
 
-   ./document_loaders/examples/*
+   ./document_loaders/examples/airbyte_json.ipynb
+   ./document_loaders/examples/apify_dataset.ipynb
+   ./document_loaders/examples/aws_s3_directory.ipynb
+   ./document_loaders/examples/aws_s3_file.ipynb
+   ./document_loaders/examples/azure_blob_storage_container.ipynb
+   ./document_loaders/examples/azure_blob_storage_file.ipynb
+   ./document_loaders/examples/blackboard.ipynb
+   ./document_loaders/examples/blockchain.ipynb
+   ./document_loaders/examples/chatgpt_loader.ipynb
+   ./document_loaders/examples/confluence.ipynb
+   ./document_loaders/examples/diffbot.ipynb
+   ./document_loaders/examples/discord_loader.ipynb
+   ./document_loaders/examples/duckdb.ipynb
+   ./document_loaders/examples/figma.ipynb
+   ./document_loaders/examples/gitbook.ipynb
+   ./document_loaders/examples/git.ipynb
+   ./document_loaders/examples/google_bigquery.ipynb
+   ./document_loaders/examples/google_cloud_storage_directory.ipynb
+   ./document_loaders/examples/google_cloud_storage_file.ipynb
+   ./document_loaders/examples/google_drive.ipynb
+   ./document_loaders/examples/image_captions.ipynb
+   ./document_loaders/examples/microsoft_onedrive.ipynb
+   ./document_loaders/examples/modern_treasury.ipynb
+   ./document_loaders/examples/notiondb.ipynb
+   ./document_loaders/examples/notion.ipynb
+   ./document_loaders/examples/obsidian.ipynb
+   ./document_loaders/examples/readthedocs_documentation.ipynb
+   ./document_loaders/examples/reddit.ipynb
+   ./document_loaders/examples/roam.ipynb
+   ./document_loaders/examples/slack.ipynb
+   ./document_loaders/examples/spreedly.ipynb
+   ./document_loaders/examples/stripe.ipynb
+   ./document_loaders/examples/twitter.ipynb