docs: document Articles IV, V & VI as project-customizable in spec-driven.md#2467

Copilot wants to merge 13 commits into

maingithub/spec-kit:mainfrom

copilot/docs-spec-driven-update-articles-iv-v-vigithub/spec-kit:copilot/docs-spec-driven-update-articles-iv-v-viCopy head branch name to clipboard