Align Reference Guide and website's Architectural Concepts sections

[anno1985]

It's a bit confusing that the Architectural Concepts section on the website doesn't align with the one in the reference guide.

For example, the website goes in a lot more detail on event sourcing than the reference guide does, which is kind of odd. I would expect them to be identical (or for the reference guide to, well, reference the website, at least). :)

[smcvb]

Good, sensible points @anno1985, we'll look into aligning one another some time soon.
So, thanks for filing this issue with us!