[webapp] Add educational notes

[mwfarb]

As @matzf suggested in #15 [edit], we should provide some educational notes in context on webapp visualization features. Some users who use the webapp tool may have educated themselves and are eager to view the transparent elements of SCION. Others, may have a more opaque knowledge of its operation, and this would be a chance to educate and draw the users to deeper resources like tutorials, experiments, and the book.

Some notes that would help on each page:

• Health: Why must these checks succeed? What system requirements are at risk and why?
• Apps: We can point to tutorials here in many cases.
• As Topology: What are these services: br.., bs..., cs..., ps..? What do they do?
• ISD TRC: What are TRCs? How are they used? Where do they come from? How are cached used? What do the fields mean?
• AS Certificate: Similar questions as TRCs.