docs: add URL anchor links to encrypted images md
docs: add URL anchor links to encrypted images md

URL anchors are already generated automatically by the GH markdown
processor, but they are not visible. This adds the proper links to the
page.

Signed-off-by: Fabio Utzig <66676b1ceaf93296e098708c09494361103aa635@apache.org>

runtimeco/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,utzig/mcuboot,utzig/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,utzig/mcuboot,tamban01/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,utzig/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,utzig/mcuboot,ATmobica/mcuboot