docs/howto: Ensuring a list is a subset of another list

[jpluscplusm]

A Commented CUE guide that demonstrates this solution to the problem:

import "list"

allowed_values: ["a", "b", "c"]
my_values: [...or(allowed_values)]
my_values: ["a", "nope"]

Notes:

• it's probably worth making sure the word "subset" appears in the page's prose
• we can probably display one passing and one failing example in the same CUE, via cue eval -i
• /docs/howto/ensure-list-is-subset-of-another-list feels like a suitable path
• given this might only work 100% as expected with primitive values, it might be worth calling this out via a note and a link to proposal: allow all values to be compared for equality cue#2583

[jpluscplusm]

@sa-spag is going to give writing this guide a try 👍 🎉

[jpluscplusm]

@sa-spag Hey there 👋 Just a friendly check-in ... did you manage to get anywhere with writing this How-to Guide? :-)

[sa-spag]

@sa-spag Hey there 👋 Just a friendly check-in ... did you manage to get anywhere with writing this How-to Guide? :-)

👋 I had other priorities, but it is still in my backlog.