[WIP] Documenting the SeoBundle

Replaces https://github.com/symfony-cmf/symfony-cmf-docs/pull/438

| Q   | A
| --- | ---
| Doc fix? | no
| New docs? | yes
| Applies to | dev
| Fixed tickets | -

Todo
---

 * [ ] Update for https://github.com/symfony-cmf/SeoBundle/pull/153
 * [ ] Add information about Extras