Read all localized urls in content

Feature needed for:

* Language-Switcher
* HREF-LANG attributes in `sitemap.xml`
* HREF-LANG tags in html pages

__Related PRs:__

* https://github.com/sulu-cmf/sulu-standard/pull/374

__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/256
| BC Breaks     | none
| Doc           | https://github.com/sulu-cmf/docs/commit/f0d2613b310590cdbbb77e12953ecd2378705c38 , https://github.com/sulu-cmf/docs/commit/013b5c18f0c659e753137eaf305b708ecb273695 , https://github.com/sulu-cmf/docs/commit/4ccc71932219a032cb0d3fc486b0e1b3f5e7bfa6 , https://github.com/sulu-cmf/docs/commit/7631dceda72b97b70c64028d2e1feadb77f33ef8
I would say this is out of scope for this PR...