Move existing concept design related documentation to concepts

We plan to move the concepts from designs.a.o to the website. To lay the
foundations for this move, move documentation that was on the old wiki and
referenced from designs.a.o into `concepts/`. As these were referenced by
the designs, take them to be of equal importance to that information.

Add an index page that briefly describes what the concepts constitute,
basically topics that have been researched but not necessarily
implemented. Fix up any links that pointed to the location of the old
"concept designs".

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