Document External Vault secretless auth options

[SpectralHiss]

Fixes https://github.com/cert-manager/cert-manager/issues/6150

• Explain when to use kubernetes vs jwt auth for secretless auth
• Public clouds/generic + Openshift in-depth guide

Preview: https://deploy-preview-1397--cert-manager-website.netlify.app/docs/configuration/vault/#secretless-authentication-with-a-service-account

[jetstack-bot]

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[cert-manager-prow[bot]]

PR needs rebase.

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[inteon]

closing in favor of https://github.com/cert-manager/website/pull/1504 Thanks @SpectralHiss and @maelvls for all the work you did on this!