Add a documentation generator script integrated with the top-level Cmake

[diptorupd]

Currently, dpctl documentation build is decoupled from the rest of the build system and has only a broken integration done by optionally adding the docs folder as a sub-directory in the top-level CMakeLists.txt. The integration is broken as doc generation is dependent on the dpctl package getting installed first, and presently configure steps run before install.

I am listing various issues and possible solutions that have been investigated thus far:

• Create a post-install script for docs that runs at the end of the dpctl installation to generate docs
• Docs CMakeLists.txt should have a proper install target.
• Documentation generation targets should not be part of ALL
• Add a gen_docs script to build docs
• Make sure package is rebuilt only if any pyx file changed.