NXP backend: Update user guide and docs Readme

[pytorchbot]

Summary

This PR updates NXP backend Readmes in backend and examples directories.

Test plan

cc @robert-kalmar @JakeStevens @digantdesai

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15114

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 1 New Failure, 2 Cancelled Jobs

As of commit b5a6303 with merge base e0dda90 ():

NEW FAILURE - The following job has failed:

• pull / unittest-arm-backend-with-no-fvp (test_pytest_models) / linux-job (gh)
  backends/arm/test/models/stable_diffusion/test_vae_AutoencoderKL.py::TestAutoencoderKL::test_AutoencoderKL_tosa_FP

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• pull / unittest / macos / macos-job (gh)
  ##[error]The operation was canceled.
• pull / unittest-editable / macos / macos-job (gh)
  ##[error]The operation was canceled.

This comment was automatically generated by Dr. CI and updates every 15 minutes.

[robert-kalmar]

@psiddh - The change was already merged into release/1.0 by this PR - #14892.