Add down-sampled skymap.fits to document

[delucchi-cmu]

We have skymap files at alternative orders, as requested to support HiPS viewers. We may generate the skymap at order 8, if that's the highest order we check for at import, but then also down-sample and write these files next to the skymap.fits.

So for example: https://data.lsdb.io/hats/gaia_dr3/gaia/hats.properties

hats_skymap_order=8
hats_skymap_alt_orders=2 4 6
hats_order=6

In the root level of the catalog: https://data.lsdb.io/hats/gaia_dr3/gaia/

skymap.2.fits
skymap.4.fits
skymap.6.fits
skymap.fits