Purge stale docs and skills examples

[shreemaan-abhishek]

Part of #22.

Sweep docs/ and skills/ for stale content:

• a7 upstream / consumer-group / service-template references
• top-level upstreams / consumer_groups / service_templates config sections
• old APISIX map-style upstream nodes (e.g. "backend:8080": 1) → replace with API7 EE node-array format
• plugin-config references (after its removal)

Run make validate-skills and make test-skills.

[coderabbitai]

🔗 Related PRs

#6 - test: strengthen E2E readback assertions [merged]
#7 - docs: align mTLS skill with service model [merged]
#9 - docs: migrate traffic recipes to service model [merged]

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