[code-reflection] RFR: [docs][hat] Notes for building and running with the CUDA backend

[Juan Fumero]

This PR extends the documentation for HAT to dispatch with the CUDA backend on NVIDIA GPUs.

-------------

Commit messages:
 - [docs][hat] Notes for building and running with the CUDA backend

Changes: https://git.openjdk.org/babylon/pull/503/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=503&range=00
  Stats: 127 lines in 16 files changed: 24 ins; 10 del; 93 mod
  Patch: https://git.openjdk.org/babylon/pull/503.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/503/head:pull/503

PR: https://git.openjdk.org/babylon/pull/503