[landing] add link to download page in the header

Summary:
We need to introduce a new button in the header that links to the download page I just introduced

Depends on D12583

Test Plan:
Please see the demo video below

Reviewers: inka, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12591