Move documentation generation into Maven profile 'create-docs'

Move documentation generation into Maven profile 'create-docs'

The profile is active by default, but can be deactivated via
'-P !create-docs' on the Maven command line. This helps to speed up
'mvn install', maybe in combination with skipping tests, if we just want
to create a new set of AspectJ binaries.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>