feat: customize onboarding file name (#7866)

docs/usage/self-hosted-configuration.md

lib/config/common.ts

lib/config/definitions.ts

lib/util/git/index.ts

lib/workers/repository/init/apis.spec.ts

lib/workers/repository/init/apis.ts

lib/workers/repository/onboarding/branch/create.spec.ts

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

lib/workers/repository/onboarding/branch/rebase.spec.ts

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

...boarding/pr/__snapshots__/config-description.spec.ts.snap

...rkers/repository/onboarding/pr/config-description.spec.ts

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