Document External Vault secretless auth options (v2)

[inteon]

I reviewed https://github.com/cert-manager/website/pull/1397 and simplified the documentation, while trying to maintain as much of the valuable insights as possible.

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[inteon]

/approve /lgtm

[cert-manager-prow[bot]]

@inteon: you cannot LGTM your own PR.

In response to [this](https://github.com/cert-manager/website/pull/1504#issuecomment-2165470711): >/approve >/lgtm Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes-sigs/prow](https://github.com/kubernetes-sigs/prow/issues/new?title=Prow%20issue:) repository.

[cert-manager-prow[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: inteon, SpectralHiss

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/cert-manager/website/blob/master/OWNERS)~~ [inteon] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment