mergify: fix labels with spaces

mergify: add note about write access for proper review request

mergify: fix label do not merge, fixes failure with action

mergify: fix conflict - remove also CI label

mergify: enable it only for feature-mergify ...

This will be reverted once mergify works with the PR workflow.
One rule for each condition - base is feature-mergify.

PR workflow: add mergify ...

This config should always follow our worklof and other way around. They should both be in sync.

What can mergify do? Check PR status (labels, reviewers) and act on it. First step would be to not merge anything, just use labels and help us to keep PRs up to date. Less manual work.

If this works, we can get CI to check specific labels to trigger CI (or use commands) and other things, this needs more work.