Fixes to new kernel documentation

I've addressed @eteq's comment that we need to make it clearer how the new kernel code fits in to the existing ``make_kernel`` by re-organizing the documentation - ``make _kernel`` is now no longer mentioned.

I'm still not fully happy with the organization of the docs for this as it feels like a package within a package - the convolution stuff is currently only accessible through ``astropy.nddata``, and I'm not sure I'm happy with this, since it can actually be used without even using ``NDData``. Any suggestions for how to improve this?

One solution would be to actually split out the convolution into its own sub-package, ``astropy.convolve``, since it's such a common tool in Astronomy.

Reminder to self: the ``convolution/index.rst`` page needs a 'getting started' section with a few simple examples.

cc @adonath @keflavich @eteq