Add documentation for accessing contributor and project datasets

[BorisQuanLi]

This PR addresses part of Issue #5 (Add Support for Contributor Content).

A new documentation file, docs/contributor-datasets.md, provides a step-by-step guide for discovering and downloading datasets from the /contrib/ and /projects/ directories in the Common Crawl public data bucket.

• The new file is placed in the docs/ folder, separate from the main README.md, to avoid disrupting the existing documentation structure.
• The guide includes AWS CLI commands for listing and downloading dataset files, and a Python script for downloading, decompressing, and inspecting .paths.gz files.
• These examples are intended to help users discover available datasets and preview their contents before using cc-downloader.
• Notes on current limitations are provided.

Note:
Whether this documentation should remain as a standalone file or be merged into the main README.md or another existing documentation file is open for discussion.

Breaking Changes

None.

[BorisQuanLi]

Hi @commoncrawl team, I just wanted to check if there's any feedback or updates on this PR. Let me know if there are any changes you'd like me to make!