Backport source links from 3.x.

Existing links have been updated to use the new reST role.  In some
files, I have also made cosmetic changes to the header.
diff --git a/Doc/library/webbrowser.rst b/Doc/library/webbrowser.rst
index 74e12a2..1a27d07 100644
--- a/Doc/library/webbrowser.rst
+++ b/Doc/library/webbrowser.rst
@@ -1,4 +1,3 @@
-
 :mod:`webbrowser` --- Convenient Web-browser controller
 =======================================================
 
@@ -7,6 +6,9 @@
 .. moduleauthor:: Fred L. Drake, Jr. <fdrake@acm.org>
 .. sectionauthor:: Fred L. Drake, Jr. <fdrake@acm.org>
 
+**Source code:** :source:`Lib/webbrowser.py`
+
+--------------
 
 The :mod:`webbrowser` module provides a high-level interface to allow displaying
 Web-based documents to users. Under most circumstances, simply calling the