Add guide for when / how to best use block patterns vs blocks and variations

[fabiankaegy]

We had a great discussion about this in an internal call today. The verdict was that Patterns should be all of the visual sections of a site, regardless of how they were built. But anything that editors should be able to insert on it's own should be available through the block inserter meaning either a custom block or a block variation of a core block.

This becomes especially relevant when we want to add additional custom class names to a wrapping element in order to change the visual appearance of that section. These types of settings should always have a clear indication through the UI of how they can be built.