Users with prettier installed globally may autoformat some md files with unrelated changes if they save any other change (removing underlines, special formattings).

Add to .prettierignore as these are discovered. I don't think it's worth expanding prettier rules to accept everything we have, the point of prettier is that it is "opinionated" and does not require customization.

Confirm this is just adding to prettierignore. See D15367 and history for how it works.