[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