_content/doc: add link to GC guide from /doc index

Change-Id: Iedd8c989bd5343dc5fb7109c927a516612d0ea97
Reviewed-on: https://go-review.googlesource.com/c/website/+/417155
Reviewed-by: Jamal Carvalho <jamal@golang.org>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

_content/doc/index.html

@@ -110,6 +110,11 @@ Go support.
 Summarizes tools and methodologies to diagnose problems in Go programs.
 </p>
 
+<h3 id="gc-guide"><a href="/doc/gc-guide">A Guide to the Go Garbage Collector</a></h3>
+<p>
+A document that describes how Go manages memory, and how to make the most of it.
+</p>
+
 <h3 id="dependencies"><a href="/doc/modules/managing-dependencies">Managing dependencies</a></h3>
 <p>
 When your code uses external packages, those packages (distributed as modules) become dependencies.