Converted "Univariate Bijections" and "Multivariate Bijections" tutor…

…ials to Jupyter notebook (#99)

Summary:
### Motivation
We would like to convert the two theory tutorials from MDX to Jupyter notebook format so that they can be directly run on Colab and locally, and be converted to MDX automatically by #94

### Changes proposed
Convert:
* `/website/docs/users/univariate.mdx` to `/tutorials/univariate_bijections.ipynb`
* `/website/docs/users/multivariate.mdx` to `/tutorials/multivariate_bijections.ipynb`

Pull Request resolved: #99

Test Plan: Run these two tutorials in Jupyter notebook locally

Reviewed By: horizon-blue

Differential Revision: D34616316

Pulled By: stefanwebb

fbshipit-source-id: 1cc351ece1679744a04fc361daa97d39cef67f01