[landing] Define --sans-serif for landing

Summary:
Relevant Linear issue with full context here. Defined new --sans-serif global CSS variable using the :root pseudo-class and replaced all instances of the same font usages throughout landing ('IBM Plex Sans', sans-serif).

Read more about CSS global variables.

Test Plan: Made sure everything on landing looked the same before and after the variable was introduced. So, this included the header, footer, subscription form, read docs button, and info blocks.

Reviewers: atul, palys-swm

Reviewed By: atul

Subscribers: ashoat, Adrian

Differential Revision: https://phab.comm.dev/D4356