Help/About: Add a hover effect for links on About pages.

* Add a color that passes WCAG color contrast criteria to every link.
* Add an underline for links in credits list only on hover/focus.

Props audrasjb, ixkaito, mukesh27, melchoyce, SergeyBiryukov.
Fixes #48582.
Built from https://develop.svn.wordpress.org/trunk@46852


git-svn-id: https://core.svn.wordpress.org/trunk@46652 1a063a9b-81f0-0310-95a4-ce76da25c4cd