R syllabus: Launch preparations?

Continues #504

I think we're down to the last two items that need fixing/merging before we have something ready to launch:

- #518, the exercise for the Functions concept, is ~~currently being reviewed~~ _Update: now merged_
- #506 plus the `trees-and-tibbleations` exercise, where we accidentally stumbled into something quite advanced, beyond the scope of a dataframes introductory concept. That needs simplifying, I think.

With luck, that could all be done in the next couple of days.

Once the initial 20 concept/exercise pairs are on the Exercism website, I'd like to go through the `about.md` files and add more cross-reference links to other concepts.

Once we are ready to press the button, I guess we will need to edit `config.json`:`
- `"concept-exercises": true`
- exercises to `"status": "beta"`

Having other maintainers sanity-check it might be nice, but I don't _think_ that's possible.

@BethanyG, I've tried to avoiding dragging you into the murky details of dozens of PRs, but we may be approaching a stage you'll want to comment on?

Of course, there's lots more to do post-launch (pivoting and grouping! relational joins! correlations and linear models!). Another issue for another day.