docs: create quickstart guides for self-hosted, SDK, and AISIX Cloud

[moonming]

Parent issue: #240

Summary

Create fast path guides that let customers get a successful first request with minimal setup.

Scope

• docs/quickstart/self-hosted.md
• docs/quickstart/first-model-first-key-first-request.md
• docs/quickstart/openai-sdk.md
• docs/quickstart/anthropic-sdk.md
• docs/quickstart/aisix-cloud-managed-dp.md

Acceptance Criteria

• A platform engineer can deploy a self-hosted gateway and send a first request
• An AI agent developer can connect with OpenAI SDK and Anthropic-style clients
• A managed-data-plane quickstart exists for AISIX Cloud
• The guides focus on the real user flow, not internal implementation details

[coderabbitai]

🔗 Related PRs

#102 - docs: add docs about Introduction, Quick Start, Core Concepts, and Guides [merged]
api7/docs#1452 - docs: restructure API7 API Gateway documentation [merged]

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[moonming]

Closed by the merge of #249, which shipped the two foundational quickstarts (self-hosted, first-model-first-key-first-request) at Kong-cookbook tier. The three remaining quickstart paths (openai-sdk, anthropic-sdk, aisix-cloud-managed-dp) are tracked as a separate doc-refinement issue: #273.