doc: Improve 'scute.html' generation.
doc: Improve 'scute.html' generation.

* doc/manual/Makefile.am: Override the implicit rule for the html
generation, and copy the images into the built documentation.
--
Previously, all the images were missing from the manual.

Signed-off-by: Justus Winter <4d1bd7a5ca1cd26bee0df05b6801a6ce9a22196e@g10code.com>

gpg/scute,gpg/scute,gpg/scute