[FEATURE] Introduce content navigation groups

Still some room for improvement, e.g. #150, but that could be done later.

__Tasks:__

- [x] test coverage
- [x] gather feedback for my changes
- [x] submit changes to the documentation
- [x] document the code
- [x] add test case with multiple groups

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | none
| BC Breaks     | Instead of `NavigationItem` the new `ContentNavigationItem` should be used for the ContentNavigations
| Doc           | https://github.com/sulu-cmf/docs/commit/63b63a1bc03ae6c96502c5814bb31d0b98f04271
@turbo-ele coding standards reminder ;-) ... https://github.com/sulu-cmf/SuluAdminBundle/blame/b87e3f3b8d24759a7d25b39480803894264740a4/Admin/ContentNavigation.php#L144