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/platform.rst b/Doc/library/platform.rst
index 069993c..0b78d49 100644
--- a/Doc/library/platform.rst
+++ b/Doc/library/platform.rst
@@ -9,6 +9,10 @@
 
 .. versionadded:: 2.3
 
+**Source code:** :source:`Lib/platform.py`
+
+--------------
+
 .. note::
 
    Specific platforms listed alphabetically, with Linux included in the Unix