half-way reduce bold, addressing tc39#199

I'm removing 70% of the bold markup, with these rules

* a phrase should only be bolded once in the document
  * headers (`##+`) count as bold
  * (link text)[] count as a type of bold
* new terms are made *italic*
* rare use-case for bold: scalability
  * IMHO much of the document should be re-written to be more scannable, using more headers & bullets, but not my call :)