docs: update the design index

Signed-off-by: David Scott <dave.scott@citrix.com>
docs: add the pif-properties description to the design index

Signed-off-by: David Scott <dave.scott@citrix.com>
doc: add the udhcpd doc to the design index for now

We may want to sub-divide into
 * API descriptions
 * Design deltas from old state to new state

Let's do that later when there's more of an obvious need for it.

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #2006 from djs55/lots-of-docs

Some more doc updates