doc: fix links in installation instructions

Fixes #3051.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5683056

doc/install-source.html

@@ -19,7 +19,7 @@ This document focuses on the <code>gc</code> Go
 compiler and tools (<code>6g</code>, <code>8g</code> etc.).
 For information on how to use <code>gccgo</code>, a more traditional
 compiler using the GCC back end, see
-<a href="gccgo_install.html">Setting up and using gccgo</a>.
+<a href="/install/gccgo/">Setting up and using gccgo</a>.
 </p>
 
 <p>
@@ -226,7 +226,7 @@ If you see the "hello, world" message then Go is installed correctly.
 
 <p>
 Start by taking <a href="http://code.google.com/p/go-tour/">A Tour of Go</a>
-or reading the <a href="go_tutorial.html">Go Tutorial</a>.
+or reading the <a href="/doc/go_tutorial.html">Go Tutorial</a>.
 </p>
 
 <p>
@@ -235,18 +235,18 @@ read <a href="/doc/code.html">How to Write Go Code</a>.
 </p>
 
 <p>
-Build a web application by following the <a href="codelab/wiki/">Wiki
+Build a web application by following the <a href="/doc/codelab/wiki/">Wiki
 Codelab</a>.
 </p>
 
 <p>
-Read <a href="effective_go.html">Effective Go</a> to learn about writing
+Read <a href="/doc/effective_go.html">Effective Go</a> to learn about writing
 idiomatic Go code.
 </p>
 
 <p>
 For the full story, consult Go's extensive 
-<a href="docs.html">documentation</a>.
+<a href="/doc/">documentation</a>.
 </p>
 
 

doc/install.html

@@ -153,18 +153,18 @@ read <a href="/doc/code.html">How to Write Go Code</a>.
 </p>
 
 <p>
-Build a web application by following the <a href="codelab/wiki/">Wiki
+Build a web application by following the <a href="/doc/codelab/wiki/">Wiki
 Codelab</a>.
 </p>
 
 <p>
-Read <a href="effective_go.html">Effective Go</a> to learn about writing
+Read <a href="/doc/effective_go.html">Effective Go</a> to learn about writing
 idiomatic Go code.
 </p>
 
 <p>
 For the full story, consult Go's extensive 
-<a href="docs.html">documentation</a>.
+<a href="/doc/">documentation</a>.
 </p>