WIP: Updates to the top page (DO NOT MERGE)

[ianlewis]

• New intro text.
• New Tetris image.
• Features: Container-native security, Resource Efficiency, Platform Portability.

[ianlewis]

Staged at: https://staging-positioning-dot-gvisor-website.appspot.com

[ianlewis]

This will likely be re-done from scratch. Closing for now.