feat: Add date metadata to all markdown files

- Added optional 'date' field to YAML front matter for original creation dates
- Updated build.py to read date field and use it for sorting/display
- Added display_date property to Page class (uses date if set, else modified_date)
- Backfilled dates to all 193 markdown files using git history
- Added front matter to 8 files that were missing it

Dates are sourced from git log --follow to track file renames.

Author: elimelt