Improve Docs: Add tutorial for detecting and characterizing (quasi-) periodic signals

[dhuppenkothen]

I'm wondering if more generally, it'd be helpful to have something akin to the "science threads" that e.g. Chandra has for CIAO. I think we implicitly already do in places (I think e.g. the pulsar and modeling documentation are set up in part like that), but I think we could use some more of these. In particular, having something that explains (Q)PO detection in more detail would be helpful (and could be useful for future tutorials/lectures we give, and accompany that chapter we're supposed to write).