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/basehttpserver.rst b/Doc/library/basehttpserver.rst
index 4662b4f..98b5ce4 100644
--- a/Doc/library/basehttpserver.rst
+++ b/Doc/library/basehttpserver.rst
@@ -18,6 +18,10 @@
    module: SimpleHTTPServer
    module: CGIHTTPServer
 
+**Source code:** :source:`Lib/BaseHTTPServer.py`
+
+--------------
+
 This module defines two classes for implementing HTTP servers (Web servers).
 Usually, this module isn't used directly, but is used as a basis for building
 functioning Web servers. See the :mod:`SimpleHTTPServer` and