Remove mallard page

We have a page which provides a workflow for writing documentation in
Mallard. Whilst we provide existing, mainly Gnome documentation that is
written in Mallard on the SDK, we do not appear to have written any
documentation ourselves in this format.

This information isn't really relevant to Apertis, get rid of it.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>