REF: extract classes in pandas/core/describe.py
- [ ] xref #36833
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

Extract classes ``SeriesDescriber`` and ``DataFrameDescriber``.
In the next steps I am going to enable strategy pattern and move corresponding describe functions to the concrete strategy classes (for each datatype).