fix(docs): exclude single underscore symbols from mkdocstrings

Add filter pattern to prevent single underscore symbols
from appearing in generated documentation. This cleans up
documentation output by excluding internal placeholder symbols.

Author: liblaf

template/.config/copier/mkdocs.yaml.jinja

@@ -138,6 +138,7 @@ plugins:
           filters:
             - "!__all__"
             - "!^_[^_]"
+            - "!^_$"
           summary: true
           # Docstrings
           docstring_style: google