Numpy & ReST docstrings: Remove/Ignore "optional" as types

[Masara]

Is your feature request related to a problem?

Currently if class attributes in Numpy or parameters in ReST have "optional" as a type, the "optional" will be parsed as a possible type.

Desired solution

Create a PR in Griffe to filter "optional".

Additional Context (optional)

More information can be found on the fourth point of this question: mkdocstrings/griffe#258