docs: fix Astro integration guide for v2.8.0

[jjaimealeman]

Summary

• Updated collection example to use the actual scaffolded blog-posts.collection.ts schema format instead of the non-existent defineCollection() API
• Added note that create-sonicjs already scaffolds the blog-posts collection (prevents users from creating duplicates)
• Fixed getBlogPostBySlug() to use client-side .find() — server-side filter[slug][equals] params are not yet implemented in v2.8.0
• Fixed getBlogPosts() to use client-side .filter() for published status
• Added note about API token management not being available yet in v2.8.0
• Added SSG rebuild note for dynamic content
• Added "API Filters Not Working" troubleshooting section
• Replaced curl sample content section with admin panel instructions (curl required non-existent Bearer token)

Context

Followed the guide end-to-end on a fresh create-sonicjs project and documented every issue encountered.

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