Document various approaches to product architecture [#2820904]

Inspired by #2820622: Continue to support variable products.

Ever since the early days people have had the following questions:
- Why do I need to create variations? What do I gain by doing that?
(You get a SKU for each attribute combination, allowing you to track stock, what was sold, give discounts to specific combinations)
- Can I avoid having variations for certain attributes?
(Yes, by placing them on the order item and exposing them on the add to cart form)
- Which attributes are suitable for an order item?
(Fabrics, materials, anything that doesn't need a SKU and represents a manufacturing option. Sizes could also work. Remember that only variations get the ajax image reload, so colors are best kept on the variation)

Basically, a ordinary product VS customizable product overview. Then links to the appropriate product / order docs with details.
In time this would be expanded to cover bundles, addons, digital products as well, once contribs exist for them.