Sync UI from kosmonautical/openhands-index-paul with show_all_labels feature preserved

Add dedicated API Access section to About page

Add REST API for programmatic access to leaderboard data

Fix nav links opening in new tab and simplify methodology text

Add methodology section and benchmark instance counts to About page

Update About page structure

Fix category names and benchmark assignments

Simplify and update About page

Remove multi-swe-bench from OpenHands Index

UI cleanup and About page updates

docs: Update descriptive text to use Average Score and Total Cost

Add Acknowledgements section crediting AstaBench

Remove submission page and add submission instructions to About page

Replace all AstaBench references with OpenHands Index

Initial OpenHands Index leaderboard based on ASTA Bench