Enum members aren't listed in API docs

[ixjlyons]

I think it'd be useful for the docs to list the members of enums, specifically Vendor. load_eit_data docs mention vendor-specific loading functions, but those don't seem to be present in the API docs so some concrete enumeration of the built-in readers would probably be helpful.

xref JOSS review: openjournals/joss-reviews#8179

[psomhorst]

Thank you for the suggestion.

#449 fixes this by expanding the documentation of the Vendor enum.

[psomhorst]

Closed with #449.