Added descriptions for the shipped datalake examples (example-datalake.html,
-datalake-dark.html, -datalake-full.html) to the High-Level type reference,
categorising the five-phase data stack as an unclustered High-Level variant.
Rename remaining "Schematic" references to diagram / diagram-design / Diagram Design across SKILL.md, onboarding.md, and style-guide.md. Rebased onto current main and bumped both plugin manifests to 2.3.5. Thanks @philyuchkoff.
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.
