The README's "Browse the live gallery" link pointed at the raw HTML blob,
which GitHub renders as source code rather than the gallery. Add a Pages
deployment workflow that publishes skills/diagram-design/assets as a static
site, and point the README at the deployed gallery (closes#33).
Add CONTRIBUTING.md covering all validation gates and contribution
workflows, CODE_OF_CONDUCT.md (Contributor Covenant 2.1), GitHub issue
templates (bug/feature), and a pull request template; add a Contributing
section to the README.