directives: Derive the discussion directive from 'important', not 'attention'

[rkdarst]

• Right now, the discussion directive is derived from attention, so is *yellow.
• Most other directives that want you to actively do something are green (derived from 'important') - and this was the idea.
• I think it makes more sense for discussion to be the "do something" color (green like exercises) rather than yellow like warning/attention. This makes that change
• Review:
  • Check out the directive gallery (https://coderefinery.github.io/sphinx-lesson/directives/#gallery) and see if discussion should rather be green, rather than yellow.
  • For reasoning of the current color scheme, read https://coderefinery.github.io/sphinx-lesson/directives/#gallery