feat: Slide template Markdown support

[matthew-carroll]

Description

Make it possible to define an entire slideshow with one or multiple Markdown files. The slideshow player loads the Markdown and dynamically generates slides at runtime.

This also implies some kind of templating system, on conjunction with a mapping from Markdown format to slide placeholders.

Requirements

• [ ] TBD

Additional Context

Once Markdown is supported, developers might make the same request for subsets of HTML, custom JSON formats, etc. Consider possible extension points along the way.