docs: revise LLM training proposal based on existing doc inventory

Rewrote the github-for-llms proposal to improve existing docs rather
than create parallel ones. Reduced from 13 new files to 3, focusing on
actual gaps: end-to-end examples, common mistakes, and a doc index.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ako