Add docs for layout primitives

[planktonic]

We have a few layout primitives that could be used together — let’s add some docs:

• list which atoms are good for layout, and link to the docs for each
• give some examples stories of common use cases for mixing these

Components:

• switcher (once [#779] Add some layout primitives (switcher) #784 is merged)
• stack (once [#779] Add some layout primitives (stack) #781 is merged)
• content