ENH: add GL11 validation for bullet list formatting in docstrings

[Sidra-009]

this PR introduces GL11 validation to detect missing blank lines before bullet lists in docstrings

this prevents a known Sphinx Napoleon rendering issue where bullet lists are incorrectly merged into paragraphs when not separated by a blank line after a colon.

the check triggers when:

• A line ends with ":"
• The next line starts with a bullet (-, * or +)
• No blank line exists between them

this improves docstring rendering consistency and documentation quality

[larsoner]

Can you update a test that would fail on main but pass on this PR?

I think it would show that this is not working yet, as I tried it on a repo and got:

____________________________________________________________________________ test_docstring_parameters _____________________________________________________________________________
mne/tests/test_docstring_parameters.py:190: in test_docstring_parameters
    incorrect += check_parameters_match(method, cls=cls, where=name)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
mne/tests/test_docstring_parameters.py:131: in check_parameters_match
    for err in validate(name)["errors"]
               ^^^^^^^^^^^^^^
../numpydoc/numpydoc/validate.py:755: in validate
    errs.append(error("GL11"))
                ^^^^^^^^^^^^^
../numpydoc/numpydoc/validate.py:219: in error
    return code, ERROR_MSGS[code].format(**kwargs)
                 ^^^^^^^^^^^^^^^^
E   KeyError: 'GL11'