Use tabs for variations of exercises

### How could the content be improved?

I figured out that while `challenge` blocks cannot be inside `tab`s we can put `tab` inside `challenge:

It would look like this (example on teaching practice in [teaching is a skill] )

To see live example of tabs, see [sandpaper docs](https://carpentries.github.io/sandpaper-docs/aio.html#tabbed-content)

Example uses:
- (easy) co-located/distributed versions of teaching exercises
- (medium effort) streamlining the option for the workshop website exercise
- (future) showing short-on-time options for some exercises
- any other exercises that trainers have variations for, we can host them as alternatives side by side instead of as a long list

Pros:
- more clear visual indication in rendered website by hiding other steps
- more clear indication in the source by promoting use of headings instead of If/then in English

Limitations:
- only source improvements will translate to the etherpad, will not clarify the teaching exercises

Cons:
- work to do
- maybe more I cannot think of?