[landing] Add SVG illustrations

Summary:
Add SVG illustrations exported from Figma. These will be replaced with the animated Lottie assets in subsequent diffs.

These SVG illustrations can be considered "placeholders" while working on CSS grid layout.

Test Plan: Images were visible on landing page (checked Chrome/Safari/Firefox)

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, palys-swm, Adrian

Differential Revision: https://phabricator.ashoat.com/D1014