Generate a docs table for file format examples

[annehaley]

Resolves #1489

• Create a new pooch datastore to fetch format example files
• Write a function to open format examples and evaluate table columns
• Write a function to save results to disk as an RST-formatted table
• Invoke the file fetching and table generation as a step in mkdocs.sh
• Copy parakon hosted files to a more accessible location
• Confirm that all format examples are public, small enough to download during CI, multiframe if possible, and visually interesting

[manthey]

Overall this is great.

[manthey]

Do you think it would be possible to make the format column have both a link and a tooltip using custom html? That is

.. raw:: html

   <a href="{reference_link}" title={long_name}>{col_value}</a>

should work (embedded in the rest of the table cell appropriately).

[manthey]

I'd love to figure out the tooltip-and-link rst so we can not have the word (spec) repeated in each row, but that could be a different PR.

I also wonder if we want to find or generate more relevant (e.g., either WSI or geospatial) samples for some of the download examples, but that, too, can be a different PR.

And, I note that a minor change in the PIL source will let it show GIF as multi-frame (the first image was palette mode which meant that we decided the image stack wasn't consistent enough to show the frames; see a different PR for handling this).