Move submission checklist from designs.a.o to policies

We have a stub in guidelines for the contribution checklist. Move the
content over from designs.a.o and move to policies.

Add in alias for `old-designs` to record the location used on
designs.a.o that we can be used once we've dropped all the contents
of designs.a.o as a redirect to the website.

Flag file to be generated as PDF since it's a document from
designs.a.o.

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