[landing] Reduce "Read the docs" font-size at smallest breakpoint

Summary: Reduce the font-size of "read the docs" text so it remains on a single line at narrowest breakpoint.

Test Plan: Looks as expected on Safari/Chrome/Firefox. Stays on single line at narrowest breakpoint.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, palys-swm, Adrian

Differential Revision: https://phabricator.ashoat.com/D1096