Redesign documentation homepage for product discoverability

[Devon-White]

Summary

• replace the documentation homepage with a theme-aware SignalWire command center
• add real tabbed Server SDK, REST, SWML, and Browser SDK examples
• organize product and interface discovery with Fern cards and contextual destinations
• provide a concise llms-only homepage index while excluding the visual layout from LLM exports
• align homepage CTAs with the SignalWire marketing treatment in light and dark themes
• ignore local Superdesign working files

Validation

• yarn fern-md-check
• git diff --check