Add shared Hiro API infrastructure pages (keys, rate limits, headers)

[huth-stacks]

Summary

• Adds 3 shared Hiro API infrastructure pages faithfully migrated from Hiro docs
• Adds SUMMARY.md navigation with Hiro-specific APIs properly cordoned off from generic Stacks docs
• Adds self-hosted vs Hiro-hosted clarification to Stacks Node RPC page

Files Added

• docs/reference/api/api-keys.md - Full API key documentation with actual RPM thresholds, security warnings, and usage examples
• docs/reference/api/rate-limits.md - Complete rate limits with actual numbers (50 RPM unauth, 500 RPM auth), faucet limits, service type separation
• docs/reference/api/response-headers.md - Response header documentation with service classification table

Files Modified

• docs/reference/SUMMARY.md - Added "Hiro-Hosted APIs" subsection (after Stacks Node RPC, addressing reviewer's IA ordering concern)
• docs/reference/api/stacks-node-rpc/README.md - Added hint clarifying self-hosted vs Hiro-hosted distinction

Reviewer Feedback Addressed

• the reviewer "wording differs arguably destructively" on api-keys → restored Hiro wording verbatim
• the reviewer "wording differs and limits are not provided" on rate-limits → restored actual numbers (50/500 RPM)
• the reviewer IA ordering concern → Hiro-specific sections placed in their own subsection, not before Stacks Node RPC

Context

PR 2 of 7 decomposing the Hiro docs migration (#1847). Depends on PR 1 (#1850). PR 4 (remaining API sections) depends on this.

Test plan

• Verify rate limit numbers match Hiro source (50 RPM unauth, 500 RPM auth, faucet limits)
• Verify api-keys security warning is complete
• Verify SUMMARY.md ordering puts Hiro APIs after generic Stacks docs
• Run markdown link checker

🤖 Generated with Claude Code

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