clean preview page frontend (#1251)

* remove colors

* remove npm scripts

* remove build options from next.config.js

* remove from gitignore

* remove from global.d.ts

* remove global scroll behavior

* remove from nextjs pre render manifest type

* remove pattern from sitemap exclude list

* remove i18n

* remove preview store

* remove preview enabled flags list

* remove /preview route

* remove PREVIEW const

* remove preview data from plugin context

* remove preview fixture

* remove preview code from root app components

* remove preview code from MDX route

* remove preview click away hook

* remove preview related code from src/components/

* remove preview docs