Support for rendering methods section of docstring (or explicit error)

[machow]

Currently, we do not support the rendering of a custom methods section in a numpydoc style docstring. Here is its description in the numpydoc spec:

It seems like--similar to the attributes section--the methods section often conflicts with just documenting the methods themselves. For example, quartodoc can summarize your methods using their docstrings.

Currently, including a methods section raises an error during rendering:

NotImplementedError: Unsupported type: <class 'griffe.docstrings.dataclasses.DocstringSectionFunctions'>

At the very least, we should make this message clearer (or issue a warning instead).