docs: clarify ADC constraints for 0.2.0

[dbrattli]

Summary

• Condense the quickstart guidance into a short active-constraints policy.
• Require explicit justification when changing an invariant:.
• Keep the directive-by-directive rules in the existing-comments section as the single source of truth.
• Bump the specification version from 0.1.1 to 0.2.0.

Motivation

The previous quickstart language was vague, while copying the detailed rules there would duplicate the canonical guidance. This gives adopters the core idea without maintaining the same policy in two places.

Versioning impact

Minor release. The change adds compatible guidance without changing existing directive semantics, consistent with the specification's Semantic Versioning policy.

Validation

• npx --yes markdownlint-cli2@0.23.2 --config .markdownlint.jsonc "**/*.md"
• git diff --check