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

doc/debugging_with_gdb.html

 <!--{
 	"Title": "Debugging Go Code with GDB",
-	"Path": "/ref/gdb"
+	"Path": "/doc/gdb"
 }-->
 
 <p><i>

doc/docs.html

@@ -124,6 +124,7 @@ Guided tours of Go programs.
 <ul>
 <li><a href="/doc/articles/go_command.html">About the Go command</a> - why we wrote it, what it is, what it's not, and how to use it.</li>
 <li><a href="/doc/articles/c_go_cgo.html">C? Go? Cgo!</a> - linking against C code with <a href="/cmd/cgo/">cgo</a>.</li>
+<li><a href="/doc/gdb">Debugging Go Code with GDB</a></li>
 <li><a href="/doc/articles/godoc_documenting_go_code.html">Godoc: documenting Go code</a> - writing good documentation for <a href="/cmd/godoc/">godoc</a>.</li>
 <li><a href="http://blog.golang.org/2011/06/profiling-go-programs.html">Profiling Go Programs</a></li>
 </ul>

doc/reference.html

@@ -37,11 +37,6 @@ one goroutine can be guaranteed to observe values produced by writes to the
 same variable in a different goroutine.
 </p>
 
-<h3 id="debugging_with_gdb"><a href="/ref/gdb">Debugging Go Code with GDB</a></h3>
-<p>
-Using GDB to debug Go programs.
-</p>
-
 <h4 id="subrepos">Sub-repositories</h4>
 
 <p>
@@ -62,6 +57,7 @@ Install them with "<code><a href="/cmd/go/#Download_and_install_packages_and_dep
 
 <ul>
 <li><a href="/doc/articles/c_go_cgo.html">C? Go? Cgo!</a> - linking against C code with <a href="/cmd/cgo/">cgo</a>.</li>
+<li><a href="/doc/gdb">Debugging Go Code with GDB</a></li>
 <li><a href="/doc/articles/defer_panic_recover.html">Defer, Panic, and Recover</a></li>
 <li><a href="/doc/articles/slices_usage_and_internals.html">Go Slices: usage and internals</a></li>
 <li><a href="/doc/articles/godoc_documenting_go_code.html">Godoc: documenting Go code</a> - writing good documentation for <a href="/cmd/godoc/">godoc</a>.</li>