Create notebook with all possible markdown we support

This is part of the larger work in this GH project:

https://github.com/jupyter/nbformat/projects/1

This particular task consists of creation a Jupyter notebook, using version 5.0 of the classic notebook, that contains examples (as markdown cells) of all the markdown we support. Here are some links to various other similar markdown syntaxes we should look at:

* [CommonMark](http://commonmark.org/) and its [spec](http://spec.commonmark.org/)
* GitHub's variants is documented in this [blog post](https://githubengineering.com/a-formal-spec-for-github-markdown/) and in their [spec](https://github.github.com/gfm/).

Finally, our own implementation uses [marked](https://github.com/chjj/marked) along with customizations for embedded LaTeX math using [MathJax](https://www.mathjax.org/).

The final result of this issue should be a PR to nbformat with the demo notebook.

Assigning this to @ashutoshbondre but @mpacer will help advise/guide this.