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/61693Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

doc/docs.html

@@ -70,6 +70,11 @@ A must read for any new Go programmer. It augments the tour and
 the language specification, both of which should be read first.
 </p>
 
+<h3 id="diagnostics"><a href="/doc/diagnostics.html">Diagnostics</a></h3>
+<p>
+Summarizes tools and methodologies to diagnose problems in Go programs.
+</p>
+
 <h3 id="faq"><a href="/doc/faq">Frequently Asked Questions (FAQ)</a></h3>
 <p>
 Answers to common questions about Go.