Demonstrate how to calculate pixels-per-beam

[keflavich]

There are operations that require converting from Jy/beam or similar units to pixels-per-beam, so we should show how to calculate that in the docs, e.g.:

cube = SpectralCube.read('example.fits')
beam = cube.beam
ppbeam = (cube.beam.sr / (cube.wcs.celestial.proj_plane_pixel_area()*u.deg**2)).decompose()

but there's probably a nicer way we can offer.

[keflavich]

spectral-cube has this built in, of course:
https://github.com/radio-astro-tools/spectral-cube/blob/c3a9eceb5ace5cab774fe4fce58a4a40c2b1b024/spectral_cube/base_class.py#L509