OPENDJ-2947 Create opendj-doc-generated-ref

This commit splits out the doc generation into a separate module in order to avoid the existing classpath complexities.

The new module helps to generate the reference documentation for tools
and configuration.
Moves opendj-doc-maven-plugin calls from opendj-server-legacy and
opendj-ldap-toolkit into the new module.
Removes 'docs' profile and creates 'man-pages' profile.
All fast documentation reference is artifacts are generated by default.
If activated, the 'man-pages' profile will generate all server tools
reference documentation and man pages which are embedded in debian and
rpm packages.
This operation is done as part of a profile because it is quite slow
(about 5 minuts).