[FEATURE] Load content twig extension

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes
- [x] submit changes to the documentation


__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | https://github.com/sulu-cmf/sulu-standard/issues/328
| BC Breaks     | none
| Doc           | https://github.com/sulu-cmf/docs/commit/aa59b86e0e0fa279775aacb40dcadf2e998021ad
How does the dev call this? Would it be better to pass the resource locator or absolute path and have a second argument as the type? We could make it accept either UUID or Path (this is what the PHPCRODM accepts):

e.g.

````
{{ sulu_render_structure('/footer/footer1', 'snippet') }}
````

Also not sure about the name, what if we have

````
sulu_render_structure($uuidOrPath, $type = null)

sulu_render_snippet($uuidOrPath);
sulu_render_page($uuidOrPath);
````

?