[landing] make all external links open in a new tab

Summary:
Based on some feedback I got in D8083, we should have the external links in the landing page open up a new tab

Depends on D8171

Test Plan:
Verified that every link opens up in a new tab like this:

Reviewers: atul, kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8172