CLN/DOC/TST: Categorical fixups (GH7768)

replaces #7768, #8006 
closes #3678 

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

Categorical: use s.values when calling private methods

  s.values is the underlying Categorical object, s.cat will be changed
  to only expose the API methods/properties.

Categorical: Change series.cat to only expose the API

Categorical: Fix order and na_position

Categorical: Fix comparison of Categoricals and Series|Categorical|np.array

  Categorical can only be comapred to another Categorical with the same levels
  and the same ordering or to a scalar value.

  If the Categorical has no order defined (cat.ordered == False), only equal
  (and not equal) are defined.

Categorical: Tab completition tests
for code, you should use doube backticks (``` `` ```) instead of a single one to get it rendered as code.

BTW, I think it is possible to set sphinx to render a single quoted the same as double if we would want that.