[landing] CSS Grid one-column responsive breakpoint

Summary: Single breakpoint at 1099px where CSS grid goes from having two columns to having one. Subsequent diffs will consider each of the breakpoints specified in the Figma document. This diff is mostly about using grid-template-areas to change the order in which grid cells are displayed (eg hero image before hero copy)

Test Plan: NA

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, palys-swm, Adrian

Differential Revision: https://phabricator.ashoat.com/D1031