Change sidebar for API section to view all `napari.submodule` as entries without having to manually expand the modules entry

[willingc]

📚 New content request

This isn't a new content request. Instead, I'm requesting how we display the API documentation sidebar with all the submodules present in an expanded list.

As a developer, I like to see the entire API while I search for the module, class or function that might help me. By having everything expanded like the scikit-learn docs, it's easier to do a quick look to: 1) see if there is an API entry for a topic that I have in mind; 2) get a sense of the overall API.

Outline

Expand the API sidebar by default.

[melissawm]

Good idea! Cross-referencing napari/napari-sphinx-theme#176

This may be better suited for the theme repo, too.