docs: add missing quickstart guides (openai-sdk, anthropic-sdk, aisix-cloud-managed-dp)

Background

Follow-up to #243 (closed by #249). The docs rebuild shipped two of the five planned quickstarts at the Kong-cookbook quality bar:

• ✅ docs/quickstart/self-hosted.md
• ✅ docs/quickstart/first-model-first-key-first-request.md

The other three remain as TODO and were excluded from the rebuild scope per #240's "Remaining Work Breakdown" framing ("doc refinements... separate behavior-specific issues rather than missing-section rebuild work").

Scope

Add Kong-cookbook-tier walkthroughs for the three remaining quickstart paths:

• docs/quickstart/openai-sdk.md — already exists at minimal-example tier (committed in #249); expand with Cleanup, Verify-it-works negative path, and the same observable-contract checks the other quickstarts use
• docs/quickstart/anthropic-sdk.md — same expansion against the Anthropic /v1/messages surface
• docs/quickstart/aisix-cloud-managed-dp.md — already exists as a high-level explainer; expand with a real dashboard / cert-bundle walkthrough once the AISIX-Cloud-side certificate-issuance UX is settled

Acceptance criteria

Each page must hit the bar defined by /Users/ming/.claude/skills/ai-gateway-tech-writer/references/quality-benchmarks.md:

• prerequisites, copyable commands, expected output
• observable verification (header value, status code, marker — not just 200 OK)
• cleanup step
• next steps with one-sentence link annotations

References

• Parent rebuild PR: #249
• Original quickstart issue: #243

Start by reading docs/quickstart/self-hosted.md, docs/quickstart/first-model-first-key-first-request.md, and the existing openai-sdk.md, anthropic-sdk.md, and aisix-cloud-managed-dp.md pages, then compare them with the referenced quality benchmarks. Expand each page with prerequisites, copyable commands, expected output, observable verification, cleanup, and annotated next steps; the AISIX page also requires the certificate-issuance UX to be settled.