Add documentation on existing datasets

[VasuAgrawal]

Add a new documentation page describing some existing datasets, to help improve discoverability and make it easier to compare different options.

There are a few minor things I wanted to do but wasn't able to figure out, which I'd appreciate guidance on:

• How to make a reference to the automatically generated reference/cli/ns-download-data.html, rather than hardcoding in a link to the online version (line 5).
• How to have a reference to another heading, so that the "Image Count¹" and "Image Resolution²" table headings in the summary table can directly link to the larger tables at the bottom. The standard markdown syntax didn't seem to work here, and I couldn't get the explicit targets in MyST to work.

[kerrj]

Great thank you!