spike(search): MiniSearch-backed custom docs content in header search

[thorthelegend99]

Summary

Spike: add custom docs content search using MiniSearch while preserving existing fast header search behavior.

What this changes

• adds a new API endpoint: /api/search-docs-index.json
  • indexes customPages + pages content
  • strips markdown and caps content size per entry for payload control
• keeps existing /api/search-index.json behavior for resource metadata search
• updates header search modal to:
  • lazy-load docs index only after user starts typing
  • run MiniSearch client-side against docs content
  • merge docs matches into existing header search results (Doc result type)

UX

• Search remains in the same header modal.
• No extra route required; docs content results show alongside normal results.

Perf notes

• two-index strategy (metadata + docs content)
• docs index lazy-loaded on intent (query length >= 2)
• content is pre-trimmed to avoid huge payloads

Status

• Spike / draft for discussion, ranking tuning, and payload thresholds.

Validation

• pnpm --filter @eventcatalog/core check

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Latest commit: ec5bef3

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All committers have signed the CLA.