* add doc for feature flag and experimental interfaces
* add new doc to TOC
* update pull request template to include checklist item for experimental/feature flag items
* tweak wording for experimental prefix
* fix bad formatting of bold for lines
* fix double slash in url
* add missing sentence case
* remove account signup as that is not included in Cart and Checkout block logic.
* Fix typo
Darren Ethier
Mike Jolley
Kelly Dwan