Add .DS_Store to gitignore and remove tracked files

Ignore macOS system files to keep repository clean.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: rammprasad

.gitignore

@@ -6,3 +6,4 @@ slides/rinpharma2024_sdtm_workshop.html
 slides/rinpharma2024_sdtm_workshop_files/*
 /.quarto/
 **/*.quarto_ipynb
+.DS_Store