Allow folder named different to './Documentation' to be docroot

[marble]

Meanwhile our countainer is almost a universal tool to render documentation of projects. Foreign projects however often have documentation in - for example - the ./docs folder. TYPO3 conventions say that the documentation folder MUST be name ./Documentation. It would be cool to have a commandline parameter to account for differently named folders as well.

An example for a foreign rep would be https://github.com/executablebooks/sphinx-tabs