Improvement to numba-dpex documentation

[diptorupd]

A meta issue to aggregate various numba-dpex documentation related issues in one place:

• Update getting started guide #1085
• Ensure no all code snippets in the documentation work and do not raise deprecated warning
  • Fix warnings in API docs #398
  • Fix documentation warnings  #539
  • Deprecation warning for examples in documentation #1042
• Add section for environment variables in documentation #683
• Update User Guide about compilation pipeline  #385
• Add API Reference for numba-dpex to the documentation site #1111