* refactor: signal et al. are now experimental nettests
We move signal into the experimental nettests group. While there,
also start adding dnscheck and stunreachability as well.
It seems there's more work to be done to correctly represent
the results of dnscheck, but this is fine!
The experimental section is here exactly for this reason!
In terms of UI, the new command is `ooniprobe run experimental`.
We will most likely move signal out of experimental soon, since it's
already working quite well. We need to keep it here for one more
cycle because the desktop app is not ready for it.
See the following issues:
1. https://github.com/ooni/probe/issues/1378
2. https://github.com/ooni/probe/issues/1262
* fix(dnscheck): spell check
* fix: improve documentation
