doc: rewrite references to plan9.bell-labs.com to 9p.io

The plan9.bell-labs.com site has fallen into disrepair.
We'll instead use the site maintained by contributor David du Colombier.

Fixes #14233

Change-Id: I0c702e5d3b091cccd42b288ea32f34d507a4733d
Reviewed-on: https://go-review.googlesource.com/19240Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: David du Colombier <0intro@gmail.com>

doc/asm.html

@@ -12,7 +12,7 @@ The document is not comprehensive.
 
 <p>
 The assembler is based on the input style of the Plan 9 assemblers, which is documented in detail
-<a href="http://plan9.bell-labs.com/sys/doc/asm.html">elsewhere</a>.
+<a href="https://9p.io/sys/doc/asm.html">elsewhere</a>.
 If you plan to write assembly language, you should read that document although much of it is Plan 9-specific.
 The current document provides a summary of the syntax and the differences with
 what is explained in that document, and
@@ -23,7 +23,7 @@ describes the peculiarities that apply when writing assembly code to interact wi
 The most important thing to know about Go's assembler is that it is not a direct representation of the underlying machine.
 Some of the details map precisely to the machine, but some do not.
 This is because the compiler suite (see
-<a href="http://plan9.bell-labs.com/sys/doc/compiler.html">this description</a>)
+<a href="https://9p.io/sys/doc/compiler.html">this description</a>)
 needs no assembler pass in the usual pipeline.
 Instead, the compiler operates on a kind of semi-abstract instruction set,
 and instruction selection occurs partly after code generation.

doc/codewalk/codewalk.xml

@@ -91,7 +91,7 @@
 
 	The full address syntax is summarized in this table
 	(an excerpt of Table II from
-	<a href="http://plan9.bell-labs.com/sys/doc/sam/sam.html">The text editor <code>sam</code></a>):
+	<a href="https://9p.io/sys/doc/sam/sam.html">The text editor <code>sam</code></a>):
 	<br/><br/>
 
 	<table>

doc/go_faq.html

@@ -98,7 +98,7 @@ What's the origin of the mascot?</h3>
 <p>
 The mascot and logo were designed by
 <a href="http://reneefrench.blogspot.com">Renée French</a>, who also designed
-<a href="http://plan9.bell-labs.com/plan9/glenda.html">Glenda</a>,
+<a href="https://9p.io/plan9/glenda.html">Glenda</a>,
 the Plan 9 bunny.
 The <a href="https://blog.golang.org/gopher">gopher</a>
 is derived from one she used for an <a href="http://wfmu.org/">WFMU</a>