doc: uefi: capsules: Add Capsule Update porting section

doc: uefi: capsules: Add Capsule Update porting section

Created a capsule update porting section in the documentation that outlines
the steps a board developer must do when porting from an existing reference
board implementation.

In particular, added a big warning that new capsule GUID's need to be
defined.

Signed-off-by: Jonathan Humphreys <j-humphreys@ti.com>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>