docs: link to the assembler document from the Documents tab

Fixes #8156.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/108840045

doc/docs.html

@@ -145,6 +145,7 @@ Guided tours of Go programs.
 <li><a href="/blog/profiling-go-programs">Profiling Go Programs</a></li>
 <li><a href="/doc/articles/race_detector.html">Data Race Detector</a> - a manual for the data race detector.</li>
 <li><a href="/blog/race-detector">Introducing the Go Race Detector</a> - an introduction to the race detector.
+<li><a href="/doc/asm">A Quick Guide to Go's Assembler</a> - an introduction to the assembler used by Go.
 </ul>
 
 <h4 id="articles_more">More</h4>