Nicer code callouts

• Make (or rather fake) code callout references in PDFs look consistent, even when they occur in prose rather than in the callout list
• Use callout icons in HTML
  • only works for code and actual callout list
  • (fake) callout references in prose look like ② instead
• Fix wrong numbering in source
  • (Didn't affect result, but caused warning asciidoctor: WARNING: PewPew_Cheatsheet.adoc: line 54: callout list item index: expected 8, got 7)