Create API docs for numba-dpex

[diptorupd]

We need proper API docs and improve docstrings and comments in our code. I want to create a similar API doc page like dpctl or dpnp. We have to prioritize it for next release.

• Create API docs page Create API docs #388
• Update docstrings and comments in code Update docstrings and comments in code #399

[diptorupd]

closing in favor of #1111