Add Dragonfly cache configuration documentation

[vyavdoshenko]

Purpose of this pull request

This pull request (PR) adds documentation for configuring Dragonfly as a cache backend for Adobe Commerce. Dragonfly is a modern in-memory datastore that is fully compatible with Redis APIs, offering significant performance improvements through its multi-threaded architecture.

The documentation includes:

• Main configuration guide with installation instructions (Docker and binary)
• Default cache and page cache configuration
• Session storage configuration
• Dragonfly-specific configuration flags and best practices

Affected pages

New pages (not yet published):

• help/configuration/cache/config-dragonfly.md
• help/configuration/cache/dragonfly-pg-cache.md
• help/configuration/cache/dragonfly-session.md

Updated pages:

• help/configuration/TOC.md (added Dragonfly section to navigation)

Additional information

Dragonfly does not have native Adobe Commerce CLI support, so the documentation uses Redis backend configuration (--cache-backend=redis), which is fully compatible with Dragonfly.

Key differences from Redis/Valkey documentation:

• Uses redis-cli for verification (Dragonfly compatible)
• Snapshot-only persistence (Dragonfly does not support AOF)
• Note about Redis Sentinel not being applicable (Dragonfly has its own replication mechanism)

Links to the affected code

• Dragonfly GitHub repository
• Dragonfly documentation

What's New highlights

• New topic: Configure Dragonfly - Learn how to configure Dragonfly as a high-performance Redis-compatible cache backend for Adobe Commerce.