gh-108794: doctest counts skipped tests (#108795)

* Add 'skipped' attribute to TestResults.
* Add 'skips' attribute to DocTestRunner.
* Rename private DocTestRunner._name2ft attribute
  to DocTestRunner._stats.
* Use f-string for string formatting.
* Add some tests.
* Document DocTestRunner attributes and its API for statistics.
* Document TestResults class.

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>