Add help controller

[kevinpapst]

• add a markup renderer (composer: erusev/parsedown)
• add a twig filter to render an input string as markup (see https://github.com/symfony/demo/blob/master/src/Twig/AppExtension.php)
• add a controller that can render the markup documentation files from /var/docs/
• initial display: README.md
• add a link in the control sidebar / help tab
• action parameter is the plain filename without extension (validate and throw exception)
• add integration test that checks the validation and a docu chapter

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.