Restructure Python examples

[ndgrigorian]

This PR proposes a restructuring of the Python examples, specifically the USM-related examples

These examples were each very simple scripts, all relating to various operations with USM: allocation, host-access, and copying between host and device. By consolidating them, they can be run using the same _runner construct as the other examples.

As a result of this, the Conda package workflow's run of examples is updated to use the runner construct. This also means that all of the examples are being run, instead of just the USM examples.

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpctl/pulls/2292/index.html

[coveralls]

coverage: 77.262%. remained the same — restructure-python-examples into master