feat: DroneCI (#4193)

lib/config/definitions.js

lib/manager/droneci/extract.ts

lib/manager/droneci/index.ts

lib/manager/droneci/update.ts

lib/manager/index.ts

renovate-schema.json

test/config/__snapshots__/validation.spec.js.snap

test/manager/droneci/__snapshots__/extract.spec.ts.snap

test/manager/droneci/_fixtures/.drone.yml

test/manager/droneci/extract.spec.ts

test/manager/droneci/update.spec.ts

...rkers/repository/extract/__snapshots__/index.spec.js.snap

website/docs/configuration-options.md