Self-review guidelines for maintainers

[fredrik-bakke]

Document a process for maintainers to self-review and merge pull requests when other maintainers are not available to review.

[VojtechStep]

Looks very reasonable, thanks for writing it down. For the "Complete and document the self-review process in the PR" point, I understand that you mean the author should copy the preceding checklist to a comment on the PR and check the boxes, potentially with comments. I'd say this explicitly in the docs.