Convert documentation site to Markdown

### What is the problem or limitation you are having?

The current documentation is a RestructuredText site written using Sphinx. BeeWare now uses Markdown and MkDocs.

### Describe the solution you'd like

Convert rST files to Markdown, and convert project to use MkDocs.

Once completed, assign podium.beeware.org to the documentation.

The documentation on Briefcase and Toga can be used as a template for how to set up a project.

### Describe alternatives you've considered

None. We need to be consistent in our usage of MkDocs.

### Additional context

This will bring it in line with the rest of the BeeWare projects and documentation.