📝 Add framework integration documentation (FastAPI, Starlette, Django)

Description

Create comprehensive documentation for integrating zae-limiter with popular Python web frameworks. This documentation should provide ready-to-use patterns that developers can copy and adapt to their applications.

Target Frameworks

1. FastAPI - Dependency injection, middleware, exception handlers
2. Starlette - Middleware, exception handlers (foundation for FastAPI patterns)
3. Django - Middleware, decorator patterns, async views

Acceptance Criteria

• docs/guide/framework-integration.md created with all three framework examples
• FastAPI section includes: dependency injection pattern, middleware example, custom exception handler
• Starlette section includes: middleware example, exception handler
• Django section includes: middleware class, decorator for views, async view example
• All code examples are syntactically valid Python (can be parsed without errors)
• Each framework section includes error handling with RateLimitExceeded returning HTTP 429
• Each framework section includes RateLimiterUnavailable handling with configurable behavior
• Examples use SyncRateLimiter for sync contexts and RateLimiter for async contexts
• docs/guide/basic-usage.md updated with link to new framework integration page in "Next Steps" section
• mkdocs.yml updated to include new page in nav under User Guide

Start by reviewing docs/guide/basic-usage.md and mkdocs.yml, then create docs/guide/framework-integration.md with the requested FastAPI, Starlette, and Django patterns. Ensure examples cover the specified limiter types, error handling, and HTTP 429 behavior, are syntactically valid Python, and update the guide link and navigation entry. Done means every acceptance criterion is checked.