feat: add cert-manager-webhook-regery to docs

[darioackermann]

What does this PR do?

• It adds an url to cert-manager-webhook-regery that I am maintaining.
  This means that regery.com users can now use cert-manager 🎉
• Orders the existing list alphabetically... because half of the list was the order and the other was not.. that threw me off ;)

[cert-manager-prow]

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