Export mapped variants and add README for public data dump

[bencap]

This pull request updates the MaveDB public data export script and documentation to improve clarity, add new output files, and better describe the exported data. The most significant changes are the addition of a detailed README.md describing the archive structure and file formats, the inclusion of mapped variant JSON files, and improved logging and documentation in the export script.

Documentation and Archive Structure:

• Added a comprehensive README.md in src/mavedb/scripts/resources that explains the archive contents, file formats, and how to use the exported data. This file replaces and significantly expands on the previous in-script documentation.
• The export script now writes both LICENSE.txt and the new README.md into the ZIP archive, ensuring every dump is self-describing.

Exported Data Enhancements:

• Added export of mapped variant JSON files (mapped/{urn}.mapped-variants.json) for score sets that have completed mapping, mirroring the API output. This provides detailed VRS and mapping information for each variant.
• Improved the naming and logging of the exported ZIP file to include a timestamp and the number of score sets included.

Script and Code Quality Improvements:

• Improved log messages to provide clearer progress updates and a final completion message. [1] [2] [3]

Reference to Documentation:

• The script documentation now refers users to the new README.md for details on archive contents and file formats, instead of duplicating this information in the script itself.