broken link in read me on examples/travel_app

[shehrozeee]

Describe the bug
broken link in read me on examples/travel_app prevents people form testing the example

To Reproduce
Steps to reproduce the behavior:

1. Go to https://github.com/flutter/genui/tree/main/examples/travel_app
2. Click on https://github.com/flutter/genui/blob/main/docs/run_app_with_gemini_key.md (its linked there)
3. See error (404)

Expected behavior
the md file with instructions is shown instead of the 404

[omarfarouk228]

I'm working on this. The file was moved to docs/usage/ — fixing the broken links in travel_app and simple_chat READMEs.