Adding a new episode

Description of how to add a new episode to an existing lesson, with `sandpaper::create_episode()` and by creating a new file manually. For the latter, mention where to put the episode file, YAML front matter, and how to adjust `config.yaml` to add the new episode to the lesson navigation.
Followed by a tutorial about adding typical elements and building to see what they look like.

## Existing Content

* [Creating a new episode](https://carpentries.github.io/sandpaper-docs/episodes.html#creating-a-new-episode)