website: Add red arrows to the "More" buttons at the bottom of the news/events/press sections

Arrows on the left side of a link are used in several places on the
webiste. Adding them to the "More" buttons makes them stand out a bit
and communicates that they have a different nature from the links above
them.

Reviewed by: 0mp
Differential Revision: https://reviews.freebsd.org/D45811