Fix trying to add to a not started workqueue

[g-pavlov]

What this PR does / why we need it:

Fixes occasional panic: Trying to add to a not started workqueue

github.com/gardener/docforge/pkg/jobs.(*workQueue).Add(0xc000368900, 0x15f38a0, 0xc0000bd190)
        /Users/i342793/go/src/github.com/gardener/docforge/pkg/jobs/workqueue.go:72 +0x13d
github.com/gardener/docforge/pkg/jobs.(*controller).Enqueue.func1(0x17d8760, 0xc000338b80, 0xc000368920, 0x15f38a0, 0xc0000bd190)
        /Users/i342793/go/src/github.com/gardener/docforge/pkg/jobs/controller.go:76 +0x7f
created by github.com/gardener/docforge/pkg/jobs.(*controller).Enqueue
        /Users/i342793/go/src/github.com/gardener/docforge/pkg/jobs/controller.go:70 +0x67
panic: Trying to add to a not started workqueue

Special notes for your reviewer:
The hypothesis is that download task enqueue was a go routine and did not block until done so the document worker could exit early. Occasionally that satisfied the conditions to shutdown the queues and the download tasks queue happened to be nil at the time the enqueue tries to add to the queue. The fix removes makes the document worker wait until the download task is added and overall the enqueue a bit more fault tolerant yet informative for the status of the operation.

Release note:

Fixes occasional `panic: Trying to add to a not started workqueue`