Consider directives when validating docstrings parameters

[datapythonista]

When running the docstring validation script, errors are mistakenly reported if parameters have a deprecated, versionadded or versionchanged directive.

For example, when running ./scripts/validate_docstrings.py pandas.DataFrame.mode, the next error is returned by the script:

################################################################################
################################## Validation ##################################
################################################################################

Errors found:
	Errors in parameters section
		Parameter "dropna" description should finish with "."

But the dropna parameter description does finish with a period:

dropna : bool, default True
    Don't consider counts of NaN/NaT.

    .. versionadded:: 0.24.0

The problem is that the .. versionadded:: directive is not considered.

The script should ignore the directive when validating, and shouldn't report the error in this case.

[bengineer19]

Now adding deprecated and versionchanged directives as well.