docs: Update Quickstart example to use Vitis backend

[nikiburggraf]

Description

Updated the Quickstart example to:

• Replace less-well-supported fetch_example_model with constructing a simple Keras model
• Update backend from Vivado HLS to Vitis HLS

Type of change

For a new feature or function, please create an issue first to discuss it
with us before submitting a pull request.

Note: Please delete options that are not relevant.

• Documentation update

Tests

• Tested the non-synthesis steps in a notebook using the same Conda environment that the tutorials repo suggests

Checklist

• I have read the guidelines for contributing.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have installed and run pre-commit on the files I edited or added.
• (Not applicable)I have added tests that prove my fix is effective or that my feature works.