use the same key for the filepath and basename use case

[regisphilibert]

Currently we're using two different keys which either/or to define where to save the markdown file.

Either you use basename to define the slug and the module will deduct the rest from the _content file location or filepath and we'll use this as is.

I think we should simplify the documenation by simply using one key (target or output) and test if an extension is present.
With extension: it's the filepath, use as is.
Without extension, it's the slug, deduct the filepath.