Fix MDX html parsing errors (#6949)
* Fix MDX html parsing errors
For MDX wrap JSX parsing in a fragment before sending it of to babel for
parsing and formatting. In MDX adjacent JSX tags are permitted in the
root.
An option sent to the babel parser omits printing the root fragment and
instead just prints its children.
Solves #6943
* an attempt to address JounQin's comments
* use <$>...</$> instead of <>...</>
* refactoring
