docs: update documentation to be backend-agnostic

[27Bslash6]

Summary

CacheKit has evolved from a Redis-centric caching library to a backend-agnostic caching solution. The documentation should reflect this shift.

Current State

Documentation frequently positions Redis as the primary/default backend with other backends as alternatives. With FileBackend now available and more backends planned, we should present a neutral stance.

Scope

• README.md - Update tagline and overview
• docs/README.md - Revise introduction
• docs/quickstart.md - Present multiple backend options equally
• docs/configuration.md - Reorganize to show backend selection first
• docs/api-reference.md - Ensure backend-neutral examples
• Any other docs with Redis-centric language

Guidelines

1. Lead with the abstraction: "CacheKit provides a unified caching API with pluggable backends"
2. Equal treatment: Present Redis, File, and future backends as peer options
3. Use case driven: Guide users to choose backend based on their needs, not defaults
4. Update examples: Show backend selection explicitly rather than relying on Redis defaults

Out of Scope

• Code changes (backend resolution logic is fine as-is)
• API changes