[Documentation]: Baseline profile outdated

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Page URL (type "NEW" for a new page suggestion)

NEW

### What's the documentation problem or suggestion?

Baseline profiled part seem outdated:

> To generate the baseline profile, select the benchmark build variant and run the BaselineProfileGenerator benchmark test on an AOSP Android Emulator. Then copy the resulting baseline profile from the emulator to [app/src/main/baseline-prof.txt](https://github.com/android/nowinandroid/blob/main/app/src/main/baseline-prof.txt).

There is no `benchmark` build variant, and no `BaselineProfileGenerator` in benchmark module.

Please update it.

### Code of Conduct

- [X] I agree to follow this project's Code of Conduct

Start by checking the documentation page suggested as NEW against the current project structure, especially app/src/main/baseline-prof.txt and the benchmark module. Verify whether a benchmark build variant or BaselineProfileGenerator exists, then update the instructions so they describe the current baseline-profile workflow.

Written by the indexing model from the issue text.