[landing] CSS Grid one-column responsive breakpoint
[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

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