For now, using `--test=rspec`, but we anticipate to change RSpec to be
the default testing framework for Hanami.

MiniTest is simpler, but it's less familiar to most people.
Additionally, when this was created, Hanami itself used MiniTest.
We've since switched over to RSpec, since we needed some of its more
powerful features (and it's what most of use most of the time).

Rename "Hanami Architecture" to "Hanami's Architecture", since that's
more natural.

Also, explain more about the project structure and what `apps` are.

Fix the test syntax to use RSpec's output.

Also clarify some other language.

Specifically, refer to them as "Layout templates" instead of just
"Layouts", since the latter are a Ruby class in the app.