[WIP] Bootstrap PropertyAccess component

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#6595)
| Applies to    | 2.2+
| Fixed tickets | #2130
Todo
- [x] Document write features
- [x] Render document and fix issues
- [x] Get reviewed by @bschussek 

This is the rough first version of the first part of the documentation about the new PropertyAccess component. I will work on this further the comming days. If you spot any problems or if you think something else is better, just leave a comment please.