[landing] Reintroduce hover styling for Header nav links

Summary: Bring back hover styling for Header nav links

Test Plan:
Here's how it looks:

Reviewers: def-au1t, varun, benschac, palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3393