Add a "User Experience" section to MAINTAINERS.txt

In Drupal 7, we saw the formation of a UX team to lead major usability initiatives and help develop a holistic approach to maintaining Drupal's user experience. I think it'd be good to reflect this in MAINTAINERS.txt as important as we do other elements such as documentation, translations, major core subsystems, etc.

In my view, the major players here who've led a bunch of the discussions around this are Bojhan and yoroy. There are lots of other people who've been a tremendous help too, including catch, Gábor, and Mark and Leisa, but I don't know how they feel getting random people pinging them about user experience stuff. ;)