Feature request: Use a config variable to control generation of docs

[kennyweiss]

BLT provides a variable ENABLE_DOCS to control when docs are built. It would be great if devil_ray used that variable (or perhaps a corresponding DRAY_ENABLE_DOCS) to allow users to enable/disable docs generation.

See:

• https://github.com/LLNL/devil_ray/blob/91c3e485d73b8899e2a00b2e2d8202cb1088133b/src/CMakeLists.txt#L89
• https://github.com/LLNL/blt/blob/aea5fbf046e122bd72888dad0a7f97a07b9ff08d/cmake/BLTOptions.cmake#L14