docs: update llms.txt and remove build script

The machine-readable llms.txt and llms-full.txt files are no longer
hosted statically in this repository. They are now automatically
generated and hosted on the adk.dev documentation site. This updates
the text files to redirect to the new URLs and removes the obsolete
build_llms_txt.py script.

Closes #6108

Co-authored-by: Haran Rajkumar <haranrk@google.com>
PiperOrigin-RevId: 933317323

Author: haranrk