Improvement to numba-dpex documentation

[diptorupd]

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

• [x] #1085
• [x] Ensure no all code snippets in the documentation work and do not raise deprecated warning
  • [x] #398
  • [x] #539
  • [x] #1042
• [x] #683
• [ ] #385
• [ ] #1111