Add specific section in contributing docs for adding a new action

[jni]

... And making it accessible from the menu.

📚 New content request

Some issues in napari, such as napari/napari#7318, are not too bad if you know specifically what you need to do:

• define a function
• define an action (including keybinds)
• add a menu item
• ... profit? I forget 😅

This is actually a very quick and simple way to add functionality to napari, so we should make it as easy as possible for contributors to do this!