docs: migrate website to Gatsby v2 (#1623)

* website: initial conversion to gatsby v2

* fix unexpected history use warning

* use commonjs only to fix gatsby error

* fix gatsby import error with sidecar

* remove unused postcss-colour-functions

* remove unused prop

* lowercase layout filename import to match actual file

* chore(lint): format code

website/gatsby-browser.js

@@ -1,7 +1,6 @@
-import SmoothScroll from 'smooth-scroll';
-
 // Make scroll behavior of internal links smooth
 exports.onClientEntry = () => {
+  const SmoothScroll = require('smooth-scroll');
   new SmoothScroll('a[href*="#"]', {
     offset() {
       return document.querySelector('#header').offsetHeight;