feature: include non-local superclasses in mermaid inheritance diagram

### Is your feature request related to a problem? Please describe. I just tried out the new Mermaid inheritance diagrams - thanks for implementing them :) I noticed that they only include the super classes that are part of the current documentation, unlike the show_bases option. ### Describe the solution you'd like The inheritance diagrams should include at least all bases mentioned by show_bases. External superclasses, which have further superclasses, could be included fully or truncated at the first external parent with a ... marker showing that something was truncated.