update available activities in a table format

[yibeichan]

reviewer C:

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The reproschema-library/activities on GitHub desperately needs a README with a table saying what is in each directory.

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changes made:

1. Created a Python script (scripts/update_activities_table.py) that:

• Scans the activities directory for assessment tools
• Extracts descriptions from each activity's schema file (prioritizing description["en"] > prefLabel["en"] > activity name)
• Generates a formatted markdown table with links to each activity
• Updates the README.md file directly under the "Available Activities" section

2. Set up a GitHub Action (.github/workflows/update-activities.yml) that:

• Runs automatically when new activity directories are added
• Can be triggered manually when needed
• Updates and commits changes to README.md

3. Modified the README.mdso that the auto-generated table will be maintained with:

• Links to each activity
• Descriptions from schema files
• Last update timestamp