Clarify the philosophy and intended uses for engines

[villander]

Ember Engines just want to separate a big app into many apps manually to isolation concerns
& responsibilities, one type of microservices on front-end with emberJS.

the docs need to make sure to de-emphasize code-splitting / lazy-loading a bit
I think engines have been overused for this because they have been the only way to achieve it
and this will change with embroider