update external_cpp_references

[2bndy5]

• strips the global scope operator from confval's keys
• updates doc to use autoclass (obsoletes ignored nitpicky error)

This partially addresses the observations in https://github.com/jbms/sphinx-immaterial/issues/115#issuecomment-1172809791

[jbms]

This change looks good since we are already stripping leading ":" in _strip_template_arguments. However, I'm rather confused why we weren't getting nitpick warnings before.

[2bndy5]

Nitpick warnings about what exactly? The only nitpick warning this relieves is the one about the python class.

[jbms]

Nitpick warnings about what exactly? The only nitpick warning this relieves is the one about the python class.

Oh sorry, I was getting confused between external_cpp_references and the nitpick warnings on namespaces.