Document Fix CI/CD Pipeline flow stall troubleshooting

What does this MR do and why?

Adds a troubleshooting entry to the Fix CI/CD Pipeline Flow docs for the case where the flow starts but never finishes. It covers the three causes identified so far: no eligible runner, exhausted compute minutes (which also removes hosted runner access, making it look like a runner problem), and top-level group IP restrictions blocking hosted runners.

🔗 References

• #608249 (closed)