internal/tour: include _content/images/icons

Some icons (e.g. dark mode support icons) were not included
when running the stand-alone tour. Include them.

Change-Id: I714b60077229129d1c48237f918fc1ad16be0460
Reviewed-on: https://go-review.googlesource.com/c/website/+/527081
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Russ Cox <rsc@golang.org>

content.go

@@ -25,6 +25,7 @@ var embedded embed.FS
 
 //go:embed _content/favicon.ico
 //go:embed _content/images/go-logo-white.svg
+//go:embed _content/images/icons
 //go:embed _content/js/playground.js
 //go:embed _content/tour
 var tourOnly embed.FS