doc: move gdb doc back to /doc/ and de-emphasize on references page

Fixes #3399.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5919044
diff --git a/doc/debugging_with_gdb.html b/doc/debugging_with_gdb.html
index 1ac1c1f..b268391 100644
--- a/doc/debugging_with_gdb.html
+++ b/doc/debugging_with_gdb.html
@@ -1,6 +1,6 @@
 <!--{
 	"Title": "Debugging Go Code with GDB",
-	"Path": "/ref/gdb"
+	"Path": "/doc/gdb"
 }-->
 
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