docs: expand environment customization guide

[lbajsarowicz]

Summary

• Rewrites the "Customizing An Environment" page to fully document the .warden/warden-env.yml override mechanism
• Removes the "forthcoming" disclaimer that's been there since the page was created
• Removes outdated version: "3.5" from Docker Compose examples

What's new

• Docker Compose Overrides section explaining merge precedence (loaded last = highest priority)
• OS-specific overrides (.warden/warden-env.darwin.yml, .warden/warden-env.linux.yml)
• Custom Docker image example (overriding php-fpm image)
• Adding extra services example (adminer with Traefik routing)
• warden env config tip for previewing the merged configuration
• Advanced: Project-Level Environment Partials documenting the .warden/environments/ directory structure and full search order
• Updated Multiple Domains to use a relative link instead of hardcoded URL

Existing content preserved

• .env variable customization (PHP_VERSION, service versions, toggles)
• PHP config override example (updated syntax)
• Nginx config override example (updated syntax)
• Magento 1/2 specific customizations (unchanged)

Test plan

• Build docs with make html and verify rendered output
• All internal links resolve correctly
• Examples are valid YAML without deprecated version key

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