Warning / linting for zero-width space used in contributing markdown tutorial

Or maybe consider replacing it with another more obviously wrong character (e.g. ꙮ) that gets stripped out in the same way and will be easier to catch.

Background: the contributing tutorial uses a zero-width space heavily to intentionally break rendering of specific components (box titles, etc) because we needed to freely mix rendered and non-rendered boxes/commands/links/etc that would still be appear correct visually. `{% raw %}` was insufficient in some places (any plugin that does a `.gsub`)