Add 'architecture guidance' section to the docs

[pimvernooij]

We use several architecture design principles in our projects, that we could/should add to the docs.

These would be 'generic' best practises when building MACH systems - as even within MACH composer, you can do stuff in a lot of different ways.

[tleguijt]

Shall we move the 'Security' section into a 'Best practises' and also include a separate section in there for the Architecture