Update to reference new lesson template/styles

### How could the content be improved?

The [lesson incubation section](https://carpentries.github.io/instructor-training/15-carpentries.html#setting-out-on-your-own-together-lesson-incubation) refers to our old template/styles.

### Which part of the content does your suggestion apply to?

https://carpentries.github.io/instructor-training/15-carpentries.html#setting-out-on-your-own-together-lesson-incubation

Start at the lesson incubation section linked in the issue and identify the old template and styles it references. Compare those references with the current lesson template and styles; the work is done when the section points to the new versions.

Written by the indexing model from the issue text.