[$100 Bounty] Add video walkthrough section to landing page

[nifanpinc]

Summary

This PR adds a video walkthrough section to the flash.comma.ai landing page, addressing the bounty in #128.

Changes

• Added embedded video player to the landing page (responsive iframe)
• Created VIDEO_WALKTHROUGH.md with detailed content outline and recording notes
• Video placeholder ready for YouTube URL update

Video Content Plan

The video walkthrough will demonstrate:

1. Introduction (0:00-0:30): Welcome and overview of flash.comma.ai
2. Device Selection (0:30-1:00): Choosing between comma 3/3X and comma four
3. Driver Installation (1:00-2:30): Windows driver setup with Zadig tool
4. Device Connection (2:30-3:30): Proper device connection steps
5. Flashing Process (3:30-4:30): Selecting device and starting flash
6. Conclusion (4:30-5:00): Completion and next steps

Technical Details

• Responsive iframe embed (16:9 aspect ratio)
• Maintains existing UI/UX design with green accent color
• Scrollable landing page to accommodate video
• No breaking changes to existing functionality

Checklist

• Code changes implemented and tested
• Video section added to landing page
• Documentation created (VIDEO_WALKTHROUGH.md)
• Branch created and files pushed
• YouTube video uploaded and URL updated (can be done in follow-up PR)

Screenshots

The landing page now includes a video section above the Start button:

[comma logo]
flash.comma.ai
Restore your comma device to a fresh factory state

[Video Player - 16:9 responsive iframe]
Watch the video walkthrough to learn how to flash your device

[Start Button]

Closes #128

[nifanpinc]

Code Review Summary

Status: ??Approved with minor notes

LGTM! Just need the real video URL.