Sourcegraph 4.0 Landing Page

Created by: bretthayes

Closes #5638 (closed) and creates a new landing page for SG 4.0.

Changelog

• added sg 4.0 page
• updated vertical padding on the outer container of our ContentSection component for mobile responsiveness
• added a custom batch changes icon under /public/icons where we can keep future custom icons

Test

1. Ensure prettier has standardized the proposed changes.