Add creation dates to designs documents

The metadata in each file on designs.apertis.org didn't contain a creation
date, however this is the primary means that we are ordering the documents
by on the new Apertis website so that the newer documents are listed
first.

Whilst the documents don't contain a creation date, they are stored in a
git repository that can provide us with this information. Add the
retrieved dates into the front matter.

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