[landing] add hover effect to button

Summary: Going through the landing page one more time, and thought this would be a quick nice visual improvement with our buttons on the landing page

Test Plan:
Please see the demo videos below

Before:

After:

Reviewers: ashoat, inka

Reviewed By: inka

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12632