docs: Look for documentation in sub-directories

docs: Look for documentation in sub-directories

The ARM documentation is living in a specific folder under misc. Until
now, it was skipped during building.

In order to avoid missing future directory, use find to look for a
specific file pattern through all the directory.

Note the we already use find in other Makefile.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>