Fixes the 18 nit-picky Sphinx reference warnings in Doc/library/wsgiref.rst and removes the file from Doc/tools/.nitignore.

Where a correct target already exists, the reference is qualified so it resolves:

• The filelike object's read and close become :meth:`~io.BufferedIOBase.read``` and :meth:~io.IOBase.close```. This matches the existing ``:meth:~io.BufferedIOBase.write``` reference further down the same file.
• shift_path_info and guess_scheme are documented in wsgiref.util but were referenced from a different module context, so they are now qualified.
• serve_forever and handle_request point at socketserver.BaseServer, which is where WSGIServer actually inherits them from.

The remaining names have no documented target anywhere: Headers.keys, Headers.values, Headers.items, WSGIServer.base_environ, BaseHandler.environ and SimpleHandler.stdin / stdout / stderr are described only in prose, and paste.lint is third party. Those use the ! prefix, which keeps the semantic markup and drops the link.

I kept this to reference fixes so the diff stays limited to removing the warnings. If you would prefer the undocumented Headers methods and handler attributes to become proper .. method:: and .. attribute:: entries instead (as was done for the lzma constants in gh-151949), I am happy to do that here or in a follow-up.

Verified with a fresh nit-picky build: warnings for this file go from 18 to 0, total build warnings go from 1352 to 1334, no new warnings elsewhere, and Doc/tools/check-warnings.py --fail-if-regression --fail-if-improved exits 0.

• Issue: Fix Sphinx reference warnings in various Doc/library/ files #151950