* Rework UX guidelines to be in distinct folders and files
* Rename general guidelines to extensions guidelines
* Move contents of user-experience-guidelines.md into readme
* Rename "user experience guidelines" to "extension guidelines"
* Moved payments blurb from onboarding to readme
* Move product editor guidelines into a ux-guidelines folder
* Remove extra quote on payment guidelines image
* Add notice at top of theme customization page
* Fix linting issues
Jacklyn Biggin
Brent MacKinnon