Add short description of reprojection routines

@ellisowen and I need code to reproject Galactic diffuse emission cubes in `gammapy`.
So far we have been using Kapteyn `FITSFigure.reproject_to`, but there are installation issues with Kapteyn and it hasn't been updated in a long time, so we want to switch to use yours.

Can you add a few sentences to the docs what the difference between `regrid_cube` and `hcongrid` and `hastron` is so that people like us don't have to read the code or try them out to see which one is appropriate for which application?

(we want flux-conserving at the 1% level but don't care too much about what exactly happens to small-scale structures)