User github flavored markdown parser for mdx files

[crhistianramirez]

This would allow contributors to see what the output should look like for documentation only pages without having to manually build the project

Currently the following code will parse correctly in the project but will exclude anything after the second asterix via github. Kind of a bad example but it demonstrates that the content is being parsed differently than it is on github

`ValueExpression`: *"items.total(product.incategory('Bikes')) \*.15"*

App:

Github:

[robertsoniv]

GitHub is based off of CommonMark which is supposedly true for gatsby-plugin-mdx as well. I have a feeling these subtle differences are due to the fact that we're going from .mdx to HTML vs. plain old .md.

We could probably test this theory by adding a .md file and comparing the rendered output.