Document how the update automation works

[fwilhe]

Because it came up in a conversation

We have to manually create a comment to map the ci workflow run to the pr we create as part of the automation as we do it here. This is due to the fact that github on purpose does not trigger workflow runs for prs that are created in the automation as documented here.

The sad thing is that this does not give us this nice integration of ci results in the pr, as you can see in this manually created pr

It rather looks like this (0 checks, which is not true, but the mapping is missing):

As a workaround I've made it so we at least comment the workflow run in the pr. This seems to work reliably from what I can tell so far. It's not perfect but it's the best thing I could come up with so far.

Leaving this issue open as documentation, and maybe as a reminder to look for a better solution in the future.