content: update CSS for documentation tab
content: update CSS for documentation tab

Update CSS for the documentation view based on mocks in
go/go-discovery-gallery. This included updates to font-size, padding,
link color, and adding a border below types and functions.

Updates b/135755729

Change-Id: I0fbd74bae30cfb0d2994543eb3edd82e1322b01a
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/579451
CI-Result: Cloud Build <ea5729d4cf3392b9d382b6ca272c65aa27554f44@system.gserviceaccount.com>
Reviewed-by: Alexander Rakoczy <cc99db1321cf6c6b239cafe26d8f2eb6f93aaa8d@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite