docs(oidc): supporting dpop

[StarlightIbuki]

Description

KAG-4377

New feature support.

https://github.com/Kong/kong-ee/pull/8482

Testing instructions

Preview link:

Checklist

• [x] Review label added
• [x] Conditional version tags added, if applicable.

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[StarlightIbuki]

Should we also add a reference from Mutual TLS certificate-bound access tokens to this? @lena-larionova

[StarlightIbuki]

@StarlightIbuki I converted the example into the same format used in our other OIDC guides, which generates output for multiple tools & formats.

Also added a diagram based on info from @veenaraja, adapted to Kong - please take a look and let me know if it's accurate for our use case.

2 things are different in the use of the current design of this feature:

1. The token is not necessarily issued (proxied) by Kong;
2. We're downgrading the DPoP token thus the proof and DPoP token type are invisible from the upstream.

I will fix the diagram.

[StarlightIbuki]

@lena-larionova Hi. Could you review it again and see if the PR is ok to merge?