doc: add a diagnostics guide

Diagnostics guide lists various dimensions of
diagnostics tools and libraries available in Go.
As a follow-up, I will add an entry section where
we navigate user to the right tool depending on
the type of problem they are willing to improve
or understand better.

Change-Id: I4e94b4b834014f51c988103457da84200c7827d9
Reviewed-on: https://go-review.googlesource.com/61693
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>