Enable nightly build for doc release

[gyliu513]

Fixed llamastack/llama-stack#4315

This optimization significantly reduces CI build time by downloading and reusing pre-built documentation artifacts from llama-stack's nightly build llamastack/llama-stack#4911

Key improvements:

• Downloads pre-built docs from llama-stack repo for 'latest' builds
• Skips time-consuming API documentation generation when pre-built docs available
• Skips Docusaurus build step when pre-built site is available
• Adds comprehensive fallback mechanism when pre-built docs unavailable
• Adds build summary step to show optimization status

Benefits:

• Significant time savings by avoiding redundant API docs generation
• Maintains existing optimizations (caching, content change detection)
• Zero risk: Falls back to full build if pre-built docs are unavailable
• Only applies to 'latest' builds; version builds unaffected

Implementation details:

• Uses gh CLI to download latest nightly build artifacts
• Validates downloaded artifacts before use
• Sets PREBUILT_AVAILABLE flag to control build flow
• Multiple safety checks ensure build quality

@franciscojavierarceo @nathan-weinberg @cdoern @leseb