docs: Add comprehensive custom data guide and fix missing _component_

- Fixes #2215: Add missing _component_ in dataset examples in instruct_datasets.rst
- Addresses #2221: Create dedicated guide for using custom data
- Fixed 2 YAML examples that were missing the _component_ field
- Added comprehensive quickstart guide consolidating scattered information
- Provides examples for JSON, CSV, and HuggingFace datasets
- Includes detailed troubleshooting section
- Makes custom data setup easily discoverable for new users

This directly addresses user feedback about difficulty finding data
configuration info, providing a single authoritative source.

Author: JonSnow1807

docs/source/index.rst

@@ -129,6 +129,7 @@ torchtune tutorials.
    :hidden:
 
    basics/datasets_overview
+   basics/custom_data_quickstart
    basics/chat_datasets
    basics/instruct_datasets
    basics/multimodal_datasets