TST: suppress deprecation messages in doc tests for Index.is_foo
There were a lot of deprecation messages shown when running `ci/code_checks.sh docstrings`.

This suppresses those messages.

Example of messages that are now suppressed:

```
<doctest pandas.Index.is_boolean[1]>:1: FutureWarning: Index.is_boolean is deprecated. Use pandas.api.types.is_bool_type instead.
  idx.is_boolean()
<doctest pandas.Index.is_categorical[1]>:1: FutureWarning: CategoricalIndex.is_categorical is deprecated.Use pandas.api.types.is_categorical_dtype instead
  idx.is_categorical()
<doctest pandas.Index.is_categorical[3]>:1: FutureWarning: Index.is_categorical is deprecated.Use pandas.api.types.is_categorical_dtype instead
  idx.is_categorical()
<doctest pandas.Index.is_categorical[6]>:1: FutureWarning: RangeIndex.is_categorical is deprecated.Use pandas.api.types.is_categorical_dtype instead
  s.index.is_categorical()
```

xref #50042.
Does this fix all the warnings when validating docstrings? https://github.com/pandas-dev/pandas/issues/44642

If so, we might want to errors on warnings You’re talking about making `ci/code_checks.sh docstrings`fail when a warning is raised? This diesn’t do that, this is only about the ones that were caused directly by #50042. 
 Thanks @topper-123 