* docs - move install guide to gpdb repo

--move Install Guide source files back to gpdb repo.
--update config.yml and gpdb-landing-subnav.erb files for OSS doc builds.
--removed refs directory - unused utility reference pages.
--Also added more info to creating a gpadmin user.

These files have conditionalized text (pivotal and oss-only).

./supported-platforms.xml
./install_gpdb.xml
./apx_mgmt_utils.xml
./install_guide.ditamap
./preinstall_concepts.xml
./migrate.xml
./install_modules.xml
./prep_os.xml
./upgrading.xml

* docs - updated supported platforms with PXF information.

* docs - install guide review comment update

-- renamed one file from supported-platforms.xml to platform-requirements.xml

* docs - reworded requirement/warning based on review comments.