[josegonzalez]

Each catalog example now publishes requires, the tasks to apply before it in apply order, so the required tasks followed by the example make a complete recipe, and each reference page prints them with the example. Most of the list is derived from the task's plugin requirements and objects: a dokku_plugin for each plugin it needs, then dokku_app or dokku_service_create for each object it names but does not create, each after what it needs in turn, and sdk.TaskRequires exposes that derivation with sdk.TaskTypeOf to name the results. Examples that need more declare it: a deployed app for dokku_service_link, dokku_letsencrypt and dokku_app_clone, HTTP auth for the dokku_http_auth_* tasks, the storage entry, network and node profile the mount, network property and node sysctls examples name. The example integration tests now set up from requires, so exampleIntegrationPolicy keeps only what a task cannot express: the k3s, letsencrypt and docker-link environment gates, the skips, git_from_archive's fresh app per example, and the placeholder rewrites for certs, registry auth, the maintenance tarball and the letsencrypt DNS record. The setup-dokku action now pins dokku-postgres, which the postgres examples require, so they run in CI instead of skipping. requires is an additive key, so the catalog stays at version 1.

Closes #652