Add activity item counts and improve table formatting in README

[yibeichan]

related to #89

Changes

1. Enhanced activity table with item counts

• Added "# of Items" column showing file count in each activity's "items" folder
• Display "-" for activities without an "items" folder
• Placed "# of Items" before Description for better visibility

2. Improved table formatting

• Changed table headers from italics to bold for better visibility
• Optimized column order: ID | Activity | # of Items | Description
• Center-aligned numeric columns for readability
• Removed extra whitespace between section title and table

3. Added sequential numeric IDs

• Simple numeric IDs (1, 2, 3, ...) based on alphabetical order
• Replaced previous activity names with linked text