Adding documentation for the new webspaces

[mamazu]

:warning: DO NOT MERGE THIS INTO 2.x :warning:

Q	A
Fixed tickets	-
Related PRs	sulu/sulu#7218
License	MIT

What's in this PR?

Adding documentation about the new Symfony Config Webspace structure.

Why?

Updating the XML config and explicitly stating that all files have to be in the proper location to make it easier to debug.