[WIP] [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
the dev should call this (returns current page parent content):
```
content_load(parent)
````
or (returns smart-content page parent content)
```
{% for page in content.smartContent %}
content_load(page.parent)
{% endfor %}
````