Include a more easily understood example for nullcontext

Adds a simpler and faster alternative to ExitStack for handling
single optional context managers without having to change the
lexical structure of your code.

Original patch by James Edwards.

typing.ContextManager.

- explain single use, reusable and reentrant in docs
- converted suppress to a reentrant class based impl
- converted redirect_stdout to a reusable impl
- moved both suppress and redirect_stdout behind a functional
  facade
- added reentrancy tests for the updated suppress
- added reusability tests for the updated redirect_stdio
- slightly cleaned up an exception from contextmanager

Patch by Zero Piraeus.

statement, but add a comment that clarifies the purpose of the code.

Existing links have been updated to use the new reST role.  In some
files, I have also made cosmetic changes to the header.

Issue #11647: allow contextmanager objects to be used as decorators as described in the docs. Initial patch by Ysj Ray.

informative adjunct to the docs.  Forward-port of Raymond's r86225 and
r86245 using the new source reST role added in #10334.

Add more links to Python sources where the code is short, readable and an informative adjunct to the docs.