docs/library: Add CPython docs xref to each pertinent module.

Cross-reference text/link is implemented as RST substitution, so easy to
consistently.

docs/templates/replace.inc

-.. # This file is intended for global "replace" definitions.
+.. comment: This file is intended for global "replace" definitions.
+
+.. |see_cpython_module| replace::
+
+    *This module implements a subset of the corresponding* `CPython` *module,
+    as described below. For more information, refer to the original
+    CPython documentation:*