Update Sets documentation so it is more easily found by 'Ask AI'

[torimwhaley]

improve sets documentation for users who are referencing show_underlying_values - meaning the want to reference the same group of metrics repeatedly. They can use sets for this. This is not easily found when using 'Ask AI' on the docs site.

https://docs.lightdash.com/references/tables#sets

[linear]

PROD-2316