Update urls for some data downloads

[icweaver]

Doc preview: https://juliaastro.org/AstroImages.jl/previews/PR62/

Switched stale urls to some other hosted alternatives. I think I was able to find exact replacements for all of them except the following, which could use some review:

1. http://www.astro.uvic.ca/~wthompson/astroimages/fits/mw-crop2-small.png (replaced with https://upload.wikimedia.org/wikipedia/commons/1/15/154-panel_Widefield_Milky_Way_Panorama.jpg)

2. http://www.astro.uvic.ca/~wthompson/astroimages/fits/antenae/green.fits (replaced with https://esahubble.org/static/projects/fits_liberator/datasets/antennae/green.fits)

3. http://www.astro.uvic.ca/~wthompson/astroimages/fits/antenae/hydrogen.fits replaced with https://esahubble.org/static/projects/fits_liberator/datasets/antennae/hydrogen.fits

These last two looked to be a bit smaller than their other color channels, so I just trimmed them to match, e.g.,

# TODO: workaround until stacked images are relocated
antred = AstroImage(download("https://esahubble.org/static/projects/fits_liberator/datasets/antennae/red.fits"))[:, begin+14:end]

so that the color stacking bit of the docs would run

[sefffal]

Thanks so much for looking into this @icweaver ! I won’t be able to review until next week, but please go ahead and merge whatever changes to the docs you think are best.

yeah, the colour channels were different sizes so I had originally cropped them to match, hence why I was rehosting them.

[icweaver]

For sure, I'll go ahead and merge this then just to get the doc builds passing again. By eye, the rough cropping I did seems to get us close enough until a nicer solution is added. Thanks William!