_content/doc: add work-in-progress gc-guide.html

Currently we're publishing links to go.dev/doc/gc-guide but these links
are broken. I'm currently working on this, but at the very least let's
publish a work-in-progress page so that the status is clearer to those
who click the link.

For golang/go#53360.
Fixes golang/go#53337.

Change-Id: Id3c6107d25ce9980e88d53c8ba6ec5b4349aa29f
Reviewed-on: https://go-review.googlesource.com/c/website/+/411954
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

_content/doc/gc-guide.html

@@ -0,0 +1,19 @@
+<!--{
+	"Title": "The Go Garbage Collector",
+	"Path":  "/doc/gc-guide"
+}-->
+
+<!--
+NOTE: In this document and others in this directory, the convention is to
+set fixed-width phrases with non-fixed-width spaces, as in
+<code>hello</code> <code>world</code>.
+Do not send CLs removing the interior tags from such phrases.
+-->
+
+<p>
+<i>
+This page is currently a work-in-progress and is expected to be complete by the
+time of the Go 1.19 release. See this <a href="/issue/53360">tracking issue</a>
+for more details.
+</i>
+</p>