Add drum kit sound files, README and minor playback fixes

[KavyashreeSN]

What I changed

• Added missing sound files for the Drum Kit (sounds/*).
• Added README.md inside "01 - JavaScript Drum Kit" with usage and explanation for beginners.
• Applied playback improvements:
  • Support for numeric and character data-key mapping
  • Click/tap support for mobile
  • audio.setAttribute('preload','auto') for faster playback
  • Rewinds audio.currentTime = 0 before play()

Why

• Improves learner experience (sounds now play reliably on modern browsers and mobile).
• Documentation added for beginners who open this folder.
• Helpful console warnings added for unmapped keys to ease debugging.

Manual testing performed

• Desktop keydown + mouse click tested.
• Mobile tap tested via Live Server.
• Confirmed no console errors for existing mappings.

Notes / next steps

• If maintainers prefer character-only data-key mapping I can convert all data-key attributes to letters in a follow-up commit.
• I kept audio files compressed; happy to trim if size is an issue.

Please let me know if you want me to:

• Move the README to a different location
• Split the JS into a separate file