Add block bundle configuration

This was first part of https://github.com/symfony-cmf/symfony-cmf-docs/pull/70 and is related to: 
* https://github.com/sonata-project/SonataBlockBundle/pull/43, in bundles/block.rst:
  * added an example in the configuration section
  * mentioned in the configuration cascading explanation of the subsection _default settings_ of the section _Block Service_