Add SCVersioning to documentation

We are currently pushing the documentation to [https://usgs-astrogeology.github.io/plio/](https://usgs-astrogeology.github.io/plio/). This documentation is not versioned though. We should offer documentation that is versioned along with the source. The [SCVersioning](https://robpol86.github.io/sphinxcontrib-versioning/v2.2.1/index.html) sphinx plugin supports this.