docs: add framework integrations guide and Astro capabilities reference

[vairi]

Summary

Adds a comprehensive guide explaining that Astro Fleet is framework-agnostic and how to leverage Astro's full feature set.

Why this matters

Someone evaluating Astro Fleet might see .astro files everywhere and assume they're locked out of React/Vue/Svelte. This doc makes it clear: all 22 components are pure Astro (zero JS) by design, and any UI framework can be added alongside them when interactivity is needed.

What's in docs/framework-integrations.md

Framework integration (~60% of the doc):

• Why components are framework-agnostic and why that's the right choice
• When you need a framework vs. when you don't (decision guide)
• Step-by-step: installing React/Vue/Svelte/Solid/Preact
• Using framework components alongside Astro Fleet components (code examples)
• Client directives explained (client:load, client:idle, client:visible) with real use cases
• Using multiple frameworks on one site
• Sharing framework components across sites in the monorepo

Astro 6 capabilities (~40% of the doc):

• View Transitions (page morphing)
• Content Collections (Markdown/MDX with schemas)
• Image Optimization (automatic WebP/AVIF, CLS prevention)
• Middleware (request-level logic)
• Actions (type-safe backend functions)
• Prefetching (instant page navigation)
• CSP (Content Security Policy)
• Environment variables (Astro 6 inlining behavior)
• Architecture decision table: when to use static vs. framework components

Also updated

• README.md — "Framework-agnostic" and "Self-hosted fonts" added to What's Included; Framework Integrations in docs list; framework names in Stack line
• CLAUDE.md — framework-integrations.md added to docs references
• getting-started.md — Node.js requirement fixed (18+ → 22.12+ for Astro 6)

Test plan

• bun run build passes
• All docs render correctly in GitHub markdown
• CI passes