Slim down theme to be make our API docs more consistent with the display of pydata sphinx theme

[willingc]

I started thinking about the custom napari sphinx theme after opening napari/docs#580.

To make the API docs more usable for plugin authors and development refactors, the project would have a maintainability win. One approach that we could take:

• Remove any custom API templates and CSS to use the sensible defaults from the pydata_sphinx_theme
• Remove any unused custom templates
• Use sensible defaults for doc search in page layout

The added benefit is by staying close to the pydata CSS (other than napari brand colors) we increase our accessibility compliance.

cc/ @melissawm

[willingc]

Some screenshots (using a quick install of pydata sphinx theme without the napari custom styling):