Add better docs for new options system

[joshheinrichs-shopify]

We don't have much in the way of docs covering the new option system. There are many things in https://github.com/Shopify/cli-kit/blob/main/lib/cli/kit/opts.rb that should probably be covered, e.g.

• option
• multi_option
• position
• rest
• skippable positions
• callable defaults
• ...

Additionally, it would be good to include useful patterns such the --no-<multi_option> flag to zero a multi_option default described in #246