Adding more details to the feedback rubric in "Teaching is a skill"

In [Teaching is a skill](https://carpentries.github.io/instructor-training/11-practice-teaching/index.html) we introduce the idea of giving feedback with the 2x2 rubric (content and presentation + -) but the current lesson doesn't really describe what falls under content and what is considered presentation feedback, and learners in our recent workshop struggled with the difference. I think it would be great to have a new visual with the rubric and some examples of feedback in each quadrant, or introduce the [rubric from our teaching demos](https://carpentries.github.io/instructor-training/demos_rubric/) at this point.

Start with the “Teaching is a skill” lesson at 11-practice-teaching/index.html and compare its 2x2 rubric discussion with demos_rubric/. Decide whether the lesson needs a new visual with quadrant examples or an introduction to the existing teaching-demo rubric; done means learners can distinguish content from presentation feedback at this point.

Written by the indexing model from the issue text.