- Link to ModelAdmin showed incorrectly as `Index app` not `ModelAdmin app`
- Many code snippets were nested, causing double nesting to appear in published docs
The Menu class was not originally designed to accept menu items at constructor time (instead requiring them to be passed via hooks); ModelAdmin's SubMenu class patched this functionality in, and the documentation for extending admin views piggybacked on this. Add this functionality to the base Menu class so that we don't have this unnecessary dependency on ModelAdmin.