[3.13 fast follow] Datakit: How-to guide for auth

## Jobs to be done (optional)

Users should be able to set up internal auth using datakit, using the new features introduced in 3.12: caching, vaults, conditionals.

## Definition of done

Write a how-to guide for internal auth using datakit.

## Information

Use the OIDC guides as an example. Datakit _should_ be able to do anything OIDC can do, but for internal auth (within Kong). The client credentials flow is a popular one.

Draft from Tom: https://github.com/Kong/kong-technical-enablement/blob/main/api-gateway/3.12/datakit/README.md?plain=1 - see yaml files in the same folder for configs

https://konghq.atlassian.net/browse/KAG-7251

SME: Tom Brightbill

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