MAINT: doc: Remove doc/newdtype_example/

This example of creating a custom dtype has had only cosmetic changes
(e.g. fix whitespace, add `from __future__ ...`) since 2008, and it hasn't
been touched at all since early 2016.  It was never updated for Python 3.
The example is not well docmented, and it is not referred to anywhere else
in the documentation, so it isn't really serving a useful purpose in the
doc directory.

If someone is looking for an example of creating a custom dtype, we can
direct them to numpy/numpy/core/src/umath/_rational_tests.c.src, which, as
part of the test suite, is actively maintained.

Closes gh-9282.

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