[BB-6485] docs: automatic sprints completion

Adds docs for the automated sprint completion and fixes the bug in unflag_tickets_task.

Testing instructions

1. Clone this repo and switch to 0x29a/bb6485/document-automatic-sprint-completion.
2. docker-compose -f local.yml build
3. docker-compose -f local.yml up -d
4. docker-compose -f local.yml exec django bash
5. Run the following in the shell:

from sprintcraft.dashboard.tasks import unflag_tickets_task
unflag_tickets_task.delay(698)

6. In the output for docker-compose -f local.yml logs celeryworker you should see that the task succeeded.

Resolves #55 (closed).