Updated advance.rst:

* fixed table formatting (old version was hidden)
* fixed 'security_feed' typo, the example is actually about 'project'
* moved the RouteBuilder customisation info to its own sub-section
* moved the generic customisation text above RouteBuilder
* tidied up the generic customisation example
* tidied up comments in sort-order section so they should display better (without line wraps) on sonata website
Cleaned up and expanded batch_actions.rst:
* added note about two options when adding a new batch_action
* added note about what the merge example is trying to do and when it's available
* edited code blocks to prevent line wraps on sonata website
* updated list__batch.html.twig example to reflect latest contents
* added link to Symfony bundle extension docs