Add page about seminar as a class

[depstein]

Make a page we can put in the UW course catalog to describe the seminar. Needs to include things like:

• You probably want to be on the dub mailing list, because that's where talks are announced.
• Attendance is handled on a department-by-department basis. Talk to your department contact if you're unsure about attendance requirements.
• Seminars are often posted online a few weeks after.