Add Scalar UI support as a third documentation option

[huynguyengl99]

Hi @tfranzel ,

I'd like to suggest adding Scalar UI as a mature and user-friendly third documentation option alongside SwaggerUI and ReDoc in drf-spectacular.

Why Scalar?

• Mature & Active: 10.8k+ GitHub stars with active development
• Superior UX: Modern, clean interface that's more intuitive than traditional options
• Performance: Faster loading and smoother user interactions
• Growing Adoption: Increasingly popular in the OpenAPI ecosystem

Implementation Ready:
I've already prepared a complete implementation in PR #1425 that includes:

• SpectacularScalarView with template support
• SCALAR_UI_SETTINGS and SCALAR_DIST configuration
• Sidecar package compatibility for offline use
• Complete documentation updates and examples
• CSP troubleshooting guide

Benefits:

• Gives developers more choice in documentation UI
• Keeps drf-spectacular competitive and modern
• Zero breaking changes - purely additive feature
• Follows existing patterns for consistency

The implementation is ready to go and maintains the same high standards as the existing SwaggerUI and ReDoc integrations.

What are your thoughts on including Scalar as an official option?