DOC: fix errors/warnings in running code blocks
Cleaning up errors / warnings in the doc build, related to the ipython code blocks:

- put a section about hdf5 and Panel in verbatim code blocks + added some `:okwarning:` for Panel (will need to be properly cleaned up when Panel is removed)
- fixed cases that raised deprecation warning for `TimedeltaIndex.fillna()` with integers
- reworded case that raised deprecation warning about passing integers + timezone to DatetimeIndex
- `pd.core.config` doesn't exist anymore -> removed that call + made the actual example verbatim