Move the chores jobs to an inactive pipeline schedule

As part of gitlab#285160 (closed), this moves the chores jobs out of the regularly scheduled pipelines, and adds them to an inactive scheduled pipeline. Any time someone wants to run the chores task, they can just go to https://gitlab.com/gitlab-org/gitlab-docs/-/pipeline_schedules and click the "Play" button. Then they'll have a pipeline with only the chores jobs.

It makes use of a CHORES_PIPELINE variable that is in the scheduled pipeline config (see https://gitlab.com/gitlab-org/gitlab-docs/-/pipeline_schedules/82734/edit)

Related to gitlab#285160 (closed)