cmd/trace: add basic documentation to main page
This change adds rudimentary explanation of the various
visualizations to main page of the trace server.
There is clearly a vast amount one could write here,
especially in the form of tutorials, but I've tried to
restrict it to just basic conceptual overview.
Change-Id: Id4dfe9d47f9b31ed5f8fe39f8b3a7c60c0ae8d5a
Reviewed-on: https://go-review.googlesource.com/c/go/+/412876
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>