feat: centralise config file list (#3031)

bin/config-validator.js

docs/configuration.md

lib/platform/github/index.js

lib/workers/repository/config.js

lib/workers/repository/finalise/validate.js

lib/workers/repository/init/config.js

lib/workers/repository/onboarding/branch/check.js

lib/workers/repository/onboarding/branch/create.js

lib/workers/repository/onboarding/branch/rebase.js

lib/workers/repository/onboarding/pr/config-description.js