doc: clean up multi-image documentation

Some design.md content is causing build errors when they are included
in a .rst based documentation tree in Sphinx.

Adjust the format to make it work in both systems.

Signed-off-by: Martí Bolívar <[email protected]>

Author: mbolivar-nordic

docs/design.md

@@ -606,7 +606,7 @@ following section. The multiple image boot procedure is organized in loops which
 iterate over all the firmware images. The high-level overview of the boot
 process is presented below.
 
-+ ###### Loop 1. Iterate over all images
++  Loop 1. Iterate over all images
     1. Inspect swap status region of current image; is an interrupted swap being
        resumed?
         + Yes:
@@ -634,7 +634,7 @@ process is presented below.
             + Mark the swap type as `None`.
             + Skip to next image.
 
-+ ###### Loop 2. Iterate over all images
++  Loop 2. Iterate over all images
     1. Does the current image depend on other image(s)?
         + Yes: Are all the image dependencies satisfied?
             + Yes: Skip to next image.
@@ -643,15 +643,15 @@ process is presented below.
                 + Restart dependency check from the first image.
         + No: Skip to next image.
 
-+ ###### Loop 3. Iterate over all images
++  Loop 3. Iterate over all images
     1. Is an image swap requested?
         + Yes:
             + Perform image update operation.
             + Persist completion of swap procedure to image trailers.
             + Skip to next image.
         + No: Skip to next image.
 
-+ ###### Loop 4. Iterate over all images
++  Loop 4. Iterate over all images
     1. Validate image in the primary slot (integrity and security check) or
        at least do a basic sanity check to avoid booting into an empty flash
        area.