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

[hexylena]

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)