Move example data files to `pyregion.datasets`?

Should we move the example data files that are used in the docs (e.g. `docs/figures/pspc_skyview.fits` or `docs/figures/test.reg`) to `pyregion.datasets` to make the docs examples work for the user out of the box?

@astrofrog If you agree, this is something I could do tomorrow, it's not much work.

Start with the documentation references to docs/figures/pspc_skyview.fits and docs/figures/test.reg, then inspect how pyregion.datasets could expose those files. Confirm the intended location and packaging approach before making changes; done means the documentation examples work for users out of the box.

Written by the indexing model from the issue text.