This also helps us get rid of the sparkr-docs maven profile as docs are now built by just using -Psparkr when the roxygen2 package is available

Related to discussion in #6567

cc pwendell srowen -- Let me know if this looks better

Author: Shivaram Venkataraman <shivaram@cs.berkeley.edu>

Closes #6593 from shivaram/sparkr-pom-cleanup and squashes the following commits:

b282241 [Shivaram Venkataraman] Remove sparkr-docs from release script as well
8f100a5 [Shivaram Venkataraman] Move man pages creation to install-dev.sh This also helps us get rid of the sparkr-docs maven profile as docs are now built by just using -Psparkr when the roxygen2 package is available