Use pandoc (or similar) as Example Finder

**Describe the feature you'd like**
Instead of implementing each Example Finder by hand we could relay in ``pandoc``. It should be compatible with the finders in ``8.0.0``; the only issue would see how this affect the way that other finders can be implemented.

Nevertheless, ``pandoc`` is very complete so the probability of needing a custom finder is low.

**Describe alternatives you've considered (optional)**
``pandoc`` is the most known and there are some bindings for Python. However is not a Python native app/lib. We could research for others.