doc: update contrib.html

Fix all the local links, as contrib.html is served as
/project/

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/5656105

doc/contrib.html

@@ -12,13 +12,13 @@
 <p>View the status of Go builds across the supported operating
 systems and architectures.</p>
 
-<h3 id="roadmap"><a href="devel/roadmap.html">Roadmap</a></h3>
+<h3 id="roadmap"><a href="/doc/devel/roadmap.html">Roadmap</a></h3>
 <p>Features and ideas being developed or discussed by the Go team.</p>
 
-<h3 id="release"><a href="devel/release.html">Release History</a></h3>
+<h3 id="release"><a href="/doc/devel/release.html">Release History</a></h3>
 <p>A summary of the changes between Go releases.</p>
 
-<h3 id="release"><a href="devel/weekly.html">Weekly Snapshot History</a></h3>
+<h3 id="release"><a href="/doc/devel/weekly.html">Weekly Snapshot History</a></h3>
 <p>A summary of the changes between weekly snapshots of Go.</p>
 
 <h3 id="golang-dev"><a href="http://groups.google.com/group/golang-dev">Developer Mailing List</a></h3>
@@ -47,13 +47,13 @@ a new one.)
 We pride ourselves on being meticulous; no issue is too small.
 </p>
 
-<h3><a href="contribute.html">Contributing code</a></h3>
+<h3><a href="/doc/contribute.html">Contributing code</a></h3>
 
 <p>
 Go is an open source project and we welcome contributions from the community.
 </p>
 <p>
-To get started, read these <a href="contribute.html">contribution
+To get started, read these <a href="/doc/contribute.html">contribution
 guidelines</a> for information on design, testing, and our code review process.
 </p>
 <p>