[landing] introduce download page

Summary:
This diff introduces the download page where a user can press the button of the platform they are using to download the app on to their device.

I based the UI off of this low fidelity prototype, but very open to feedback/changing any of the copy and/or UI

Linear task: https://linear.app/comm/issue/ENG-8009/add-download-links-for-all-platforms-to-landing-page

Test Plan:
Please see the screenshots below

desktop:

tablet:

mobile:

Reviewers: ashoat, inka

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12583