Improve the Kubernetes API reference generation guide

[Caesarsage]

Updates kubernetes-api.md to the current gen-apidocs flow:

Drops GOPATH-era setup (go get, GOPATH) and the required kubernetes/kubernetes clone — the API flow is now checkout-free.

• make updateapispec → make updateapispec-enums-from-source, so the generated reference no longer silently ships without enum values.
• Documents both backends (copyapi HTML, copyapimd Markdown) and --auto-detect (no per-release config.yaml).
• Removes the stale note attributing the Markdown reference to the now-deprecated gen-resourcesdocs.

Part of #56385 .

[kubernetes-prow]

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[Caesarsage]

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[lmktfy]

Thanks!

I hope these suggestions are helpful.