Make `.stage_done` available also on docs MRs

What does this MR do?

Make the .stage_done helper job available also on docs MRs.

Why was this MR needed?

See #26593 (closed)

What's the best way to test this MR?

Create a docs-only MR based on this branch and see if it doesn't generate an invalid yaml message.

What are the relevant issue numbers?

Closes #26593 (closed)