Add section about client cert authentication for vault

[joshmue]

Documentation for https://github.com/cert-manager/cert-manager/pull/4330

Things I was not sure about whether to do:

• Add a block with an example kubernetes.io/tls Secret
• Include other optional fields mountPath and name in example

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[maelvls]

It looks good! I changed the base of your PR to the cert-manager:release-next branch so that it lands when we release 1.12.

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