modules/doc: Show nothing on index yet

Before, doc's IndexController::indexAction() showed the documentation
of Icinga Web 2 but index should rather show an overview of available
docs.

refs #4820
modules/doc: Add partial for displaying doc and toc

refs #4820
modules/doc: Rename DocParser::getDocumentation() to getDocAndToc()

refs #4820
modules/doc: Add route 'modules/doc/icingaweb' to display Icinga Web 2's documentation

refs #4820
modules/doc: Replace Parsedown's decprecated method parse() with text()

refs #4820
modules/doc: Rename populateView() to renderDocAndToc()

Rename populateView() to renderDocAndToc() since the method now sets the viewHelper
to docandtoc.phtml

refs #4820
modules/doc: Implement doc/module/$moduleName URLs

refs #4820
modules/doc: Remove icingaweb/index.phtml since it's replaced by renderDocAndToc()

refs #4820
modules/doc: Remove module/view.phtml as it has been replaced by renderDocAndToc()

refs #4820
modules/doc: List available documentations on index

refs #4820
modules/doc: Reduce tab size to two spaces in module/index.phtml

refs #4820
modules/doc: Tell which doc directory does not exist, in case it does not exist :)

Thanks Michael

refs #4820
modules/doc: Add styles

refs #4820
modules/doc: Fix toc links

The DocParser generates toc links with a hard-coded URL. This should
be changed to be generated from a route. But we did not decide how
to handle route overrides yet.

refs #4820