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/postcss.config.js

@@ -0,0 +1,19 @@
+const neatgrid = require('postcss-neat');
+const nestedcss = require('postcss-nested');
+// const colorfunctions = require('postcss-colour-functions');
+const hdBackgrounds = require('postcss-at2x');
+const cssextend = require('postcss-simple-extend');
+const cssvars = require('postcss-simple-vars-async');
+
+const styleVariables = require('./src/theme');
+
+module.exports = () => ({
+  plugins: [
+    neatgrid(),
+    nestedcss(),
+    // colorfunctions(),
+    hdBackgrounds(),
+    cssextend(),
+    cssvars({ variables: styleVariables }),
+  ],
+});