feat(gatsby-theme-docz): add jsxFrag pragma (#1269)

If a component is specifying a jsx pragma and is using `<></>` Fragment shorthand, they must also specify a fragment pragma.

https://babeljs.io/docs/en/next/babel-plugin-transform-react-jsx.html#fragments

Author: tamagokun

core/gatsby-theme-docz/src/components/Sidebar/index.js

@@ -1,4 +1,5 @@
 /** @jsx jsx */
+/** @jsxFrag React.Fragment */
 import React, { useState, useRef, useEffect } from 'react'
 import { Global } from '@emotion/core'
 import { jsx, Box } from 'theme-ui'