docs: add WLED fixture runbook

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Docs added/updated

• Added docs/wled-fixtures.md.
• Linked the new WLED fixture guide from the README developer docs list.

Codepaths covered

• WLED fixture model and runtime normalization: src/shared/ledFixtures.ts
• WLED discovery/capability routing types: src/shared/wledDiscovery.ts
• LED editor and host picker: src/renderer/pages/LedPage.tsx, src/renderer/led/*
• Main/renderer IPC wiring: src/main/engine/ipcHandler.ts, src/renderer/ipcHandler.ts
• WLED discovery, probing, UDP packet building, and runtime transport: src/main/engine/wled/*

Key knowledge gaps addressed

• How to enable the hidden WLED sidebar and configure a controller.
• How mDNS discovery and HTTP capability probing work.
• Pixel stream vs PWM routing behavior, limits, packet constraints, and RGBW fallback.
• Runtime behavior for grouping, host merging, live override, keepalives, and troubleshooting.