people: Add sections for rotation students / volunteers

[effigies]

          Could add twitter? and a section for rotation students and undergraduates/volunteers?

Originally posted by @ricardjocelyn in #35 (comment)

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Feels like it needs a @poldrack approval, but people could thumbs up/thumbs down this one.

If 👍, should they also rotate into alumni? That list would become much larger, which might justify chunking alumni into groups, as well.

[poldrack]

would there be a straightforward way to separate alumni based on a tag that denoted their role in the yml document?

I am inclined to list current rotation students but not add rotators to alumni if they don't join the lab.

I would also list current undergraduates - I think that we want a slightly higher bar for adding undergrads to the alumni list, e.g. those who write an honors thesis or contribute beyond simply helping with research for a few quarters.