feat: prHeader and prFooter (#6511)

docs/usage/configuration-options.md

docs/usage/self-hosted-configuration.md

lib/config/definitions.ts

lib/config/index.spec.ts

lib/config/index.ts

lib/config/validation.ts

lib/workers/common.ts

lib/workers/pr/__snapshots__/index.spec.ts.snap

lib/workers/pr/body/footer.ts

lib/workers/pr/body/header.ts

lib/workers/pr/body/index.ts

lib/workers/repository/init/index.ts

lib/workers/repository/onboarding/pr/index.ts