Add html anchors to design and imgtool documents

When pages are built from the markdown sources, the html anchors for
titles are automatically added, but no links are inserted in the pages.
This makes it harder to send URLs to sections; one has to browse the
page source to get the correct link. This fixes the issue by adding
links directly to the generated pages.

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