fabpot sphinx exensions as submodule

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no0)
| Applies to    | all (or 2.3+)
| Fixed tickets | https://github.com/symfony-cmf/symfony-cmf-docs/issues/522
Travis takes care of this already, so you can remove both commands (otherwise, they will be run twice). See https://github.com/symfony/symfony-docs/pull/3711