feat(tekton): Deploy to cluster

This patch introduces Tekton pipelines and Tekton triggers to the
cluster. This should provide a new, independent way to run pipelines
across various git forges.

It's a preparation of changing away from GitLab. As a first pipeline the
gitops-docs pipeline was migrated. This should provide a good example
from which further pipelines can be build.

References:
https://tekton.dev/docs/pipelines/install/
https://tekton.dev/docs/triggers/install/
https://tekton.dev/docs/pipelines/tasks/
https://tekton.dev/docs/pipelines/pipelines/
https://tekton.dev/docs/pipelines/pipelineruns/
https://tekton.dev/docs/triggers/eventlisteners/
https://tekton.dev/docs/triggers/triggers/