Add section about client cert authentication for vault

[joshmue]

Documentation for cert-manager/cert-manager#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

[netlify]

✅ Deploy Preview for cert-manager-website ready!

Name	Link
🔨 Latest commit	697486b
🔍 Latest deploy log	https://app.netlify.com/sites/cert-manager-website/deploys/64364ee8f9f77f0007b61611
😎 Deploy Preview	https://deploy-preview-1202--cert-manager-website.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site settings.

[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.

[jetstack-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: joshmue
Once this PR has been reviewed and has the lgtm label, please assign jakexks for approval. For more information see the Kubernetes Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment