Auto-update diagrams on push.

[polina-c]

We want diagrams to be refreshed after every push, by running command sh tool/diagrams.sh.

The instruction does not work because of branch protection rules. See details. After we succeed, we need to update the instruction.

Options:

1. github-push-action: ad-m/github-push-action#170 (comment)
2. https://github.com/marketplace/actions/git-auto-commit
3. peter-evans/create-pull-request: https://aiven.io/blog/how-to-automagically-open-pull-requests-with-github-actions

Pull request should not happen if there is no changes.

[polina-c]

We can experiment with creating a file with timestamp. As soon as it works, we can switch to update for diagrams.