Manage the cert-manager GitHub organisation from this repo

[inteon]

This PR adds config and tools to manage the cert-manager GitHub repo through YAML.
The code was copied from https://github.com/kubernetes/org and adapted to work for cert-manager.

TODO (future PR): figure out a strategy to keep the config in sync with https://github.com/cert-manager/community/blob/main/maintainers.csv

[cert-manager-prow]

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[inteon]

/test ?

[cert-manager-prow]

@inteon: The following commands are available to trigger required jobs:

• /test pull-org-verify

Use /test all to run all jobs.

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/test ?

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[inteon]

/test pull-org-verify