Docs: Write a guide for adding a new parser

[Hurthv]

Description

CONTRIBUTING.md has a brief explanation in the "How to Add a New Parser" section, but a detailed step-by-step guide with examples is needed.

Implementation

Create a document docs/ADDING_PARSER.md with:

1. Parser architecture (which inherits from Parse)
2. How to find selectors (tools: DevTools, selector inspector)
3. How to create a config in data/config.py
4. How to register in playcast/Parser.py
5. How to test locally
6. Examples from start to finish (for Amazon, eBay)