doc: link to new downloads page

LGTM=minux
R=golang-codereviews, minux
CC=golang-codereviews
https://golang.org/cl/102340044

doc/go1.html

@@ -2035,4 +2035,4 @@ They are available for many combinations of architecture and operating system
 Installation details are described on the
 <a href="/doc/install">Getting Started</a> page, while
 the distributions themselves are listed on the
-<a href="http://code.google.com/p/go/downloads/list">downloads page</a>.
+<a href="/dl/">downloads page</a>.

doc/install.html

@@ -6,7 +6,7 @@
 <h2 id="download">Download the Go distribution</h2>
 
 <p>
-<a href="https://code.google.com/p/go/wiki/Downloads?tm=2" id="start" class="download" target="_blank">
+<a href="/dl/" id="start" class="download" target="_blank">
 <span class="big">Download Go</span>
 <span class="desc">Click here to visit the downloads page</span>
 </a>