Remove HCatalog documentation from Hive git repo

The HCatalog documentation has been migrated to the Hive wiki a long time ago (HIVE-4993, HIVE-4994). However, the source files and the website generation properties were left in the Hive repo and they are still present under hcatalog/src/docs directory.

Ever since HIVE-4993, the docs directory is obsolete and all updates in the documentation are performed directly in the wiki (e.g., HIVE-4590, HIVE-1770).

The presence of the docs directory in the git repo adds maintenance overhead and confusion with respect to how the HCatalog documentation is updated. Moreover, it adds extra burden to ASF releases since it currently becomes part of the source code distribution.