Add BunkerWeb + MariaDB + Valkey example

[TheophileDiot]

This PR adds a new example using BunkerWeb to protect a simple web application (bunkerity/bunkerweb-hello).

Description

This example demonstrates a production-ready setup following the official BunkerWeb documentation (Full Stack / Database integration).
It includes:

• BunkerWeb (v1.6.7) as the WAF.
• MariaDB for configuration and log storage.
• Valkey (v8.0) replacing Redis for caching, configured with best practices (persistence, health checks, sysctls).
• BunkerWeb Scheduler and BunkerWeb UI (port 7000).
• Hello World App (bunkerity/bunkerweb-hello:v1.0) as the protected service.

Changes

• Added bunkerweb-mariadb/ directory with compose.yaml and README.md.
• Updated root README.md to include the new example.

Verification

• Structure follows the repository conventions.
• compose.yaml validates with docker compose config.
• Application starts and is accessible via localhost.