Add API Reference for numba-dpex to the documentation site

[ZzEeKkAa]

Currently, numba-dpex provides no API reference in the documentation page. The lack of API documentation is a serious gap in terms of usability and for new users to explore the product.

An incomplete list of things that should be added:

• [x] User API: All user-level interfaces that includes decorators, kernel API functions, supported dpnp functions
• [ ] Developer API: kernel api implementation, dpjit implementation, core