update `doc/self-hosted-runners` GitHub App instructions

[dacbd]

Cross-link: #388

[dacbd]

p0 label as instructions result in an un-usable runner

[jorgeorpinel]

Hi. What instructions does this refer to? A summary of the docs-specific task would be nice. Thanks

[jorgeorpinel]

Cross-link: #388

Should there be any more cross-links based on https://github.com/iterative/cml/labels/documentation (since that repo does not use the same A: docs label) ?

[dacbd]

the link provides the context for what is needed to update, there is an action scope that should be included when setting up the app/action used to generate ephemeral GitHub tokens https://cml.dev/doc/self-hosted-runners#app

[jorgeorpinel]

It's a pretty long conversation in that link. I think we want readable and actionable docs issues for planning, etc. Imagine someone (maybe even yourself) opens this in a year and wants to quickly understand the task and scope.

there is an action scope that should be included when setting up the app/action used to generate ephemeral GitHub tokens

So is that the summary? Should we add it to the issue description if so? What action scope is it? Where is it set up? Thanks