Update core site types

[lfades]

trafficstars

Our site types currently are Category and Brand, which are defined here: framework/commerce/types/site.ts, the brand type is currently set to any and that has to change, currently defined fields are also missing docs.

We're not exactly sure of how should we continue working with general site types, any recommendations here on how we can scale those types to work with any commerce provider is very welcome!

Every field added needs to have docs, we do that using comments, e.g:

/**
 * The checkout's id.
 */
id: string