Issue #22558: Add remaining doc links to source code for Python-coded modules.

Reformat header above separator line (added if missing) to a common format.
Patch by Yoni Lavi.

Doc/library/struct.rst

@@ -4,10 +4,14 @@
 .. module:: struct
    :synopsis: Interpret bytes as packed binary data.
 
+**Source code:** :source:`Lib/struct.py`
+
 .. index::
    pair: C; structures
    triple: packing; binary; data
 
+--------------
+
 This module performs conversions between Python values and C structs represented
 as Python :class:`bytes` objects.  This can be used in handling binary data
 stored in files or from network connections, among other sources.  It uses