Docs: Panel.dropna now exists, update docs accordingly.
ENH: PeriodIndex ops can accept Index
Merge pull request #7741 from sinhrks/period_ops

ENH/BUG: DatetimeIndex and PeriodIndex in-place ops behaves incorrectly
Merge pull request #7806 from lexual/docs_panel_has_dropna

Docs: Panel.dropna now exists, update docs accordingly.
Categorical: preserve ints when NaN are present

`Categorical([1, np.nan])` would end up with a single `1.` float level.
This commit ensures that if `values` is a list of ints and contains np.nan,
the float conversation does not take place.
Categorical: fix describe with np.nan
Categorical: ensure that one can assign np.nan
Categorical: fix assigning NaN if NaN in levels
API: change default Categorical.from_codes() to ordered=False

In the normal constructor `ordered=True` is only assumed if the levels
are given or the values are sortable (which is most of the cases), but
in `from_codes(...)` we can't asssume this so the default should be
`False`.
Categorical: add some links to Categorical in the other docs