docs: add migration guide

docs/migrate.md

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+# Migrating from PHP-FPM
+
+FrankenPHP replaces both your web server (Nginx, Apache) and PHP-FPM with a single binary.
+This guide covers a basic migration for a typical PHP application.
+
+## Key Differences
+
+| PHP-FPM setup | FrankenPHP equivalent |
+|---|---|
+| Nginx/Apache + PHP-FPM | Single `frankenphp` binary |
+| `php-fpm.conf` pool settings | [`frankenphp` global option](config.md#caddyfile-config) |
+| Nginx `server {}` block | `Caddyfile` site block |
+| `php_value` / `php_admin_value` | [`php_ini` Caddyfile directive](config.md#php-config) |
+| `pm = static` / `pm.max_children` | `num_threads` |
+| `pm = dynamic` | [`max_threads auto`](performance.md#max_threads) |
+
+## Step 1: Replace Your Web Server Config
+
+A typical Nginx + PHP-FPM configuration:
+
+```nginx
+server {
+    listen 80;
+    server_name example.com;
+    root /var/www/app/public;
+    index index.php;
+
+    location / {
+        try_files $uri $uri/ /index.php$is_args$args;
+    }
+
+    location ~ \.php$ {
+        fastcgi_pass unix:/run/php/php-fpm.sock;
+        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
+        include fastcgi_params;
+    }
+}
+```
+
+Becomes a single `Caddyfile`:
+
+```caddyfile
+example.com {
+    root /var/www/app/public
+    php_server
+}
+```
+
+That's it. The `php_server` directive handles PHP routing, `try_files`-like behavior, and static file serving.
+
+## Step 2: Migrate PHP Configuration
+
+Your existing `php.ini` works as-is. See [Configuration](config.md) for where to place it depending on your installation method.
+
+You can also set directives directly in the `Caddyfile`:
+
+```caddyfile
+{
+    frankenphp {
+        php_ini memory_limit 256M
+        php_ini max_execution_time 30
+    }
+}
+
+example.com {
+    root /var/www/app/public
+    php_server
+}
+```
+
+## Step 3: Adjust Pool Size
+
+In PHP-FPM, you tune `pm.max_children` to control the number of worker processes.
+In FrankenPHP, the equivalent is `num_threads`:
+
+```caddyfile
+{
+    frankenphp {
+        num_threads 16
+    }
+}
+```
+
+By default, FrankenPHP starts 2 threads per CPU. For dynamic scaling similar to PHP-FPM's `pm = dynamic`:
+
+```caddyfile
+{
+    frankenphp {
+        num_threads 4
+        max_threads auto
+    }
+}
+```
+
+## Step 4: Docker Migration
+
+A typical PHP-FPM Docker setup using two containers (Nginx + PHP-FPM) can be replaced by a single container:
+
+**Before:**
+
+```yaml
+services:
+  nginx:
+    image: nginx:1
+    volumes:
+      - ./nginx.conf:/etc/nginx/conf.d/default.conf
+      - .:/var/www/app
+    ports:
+      - "80:80"
+
+  php:
+    image: php:8.5-fpm
+    volumes:
+      - .:/var/www/app
+```
+
+**After:**
+
+```yaml
+services:
+  php:
+    image: dunglas/frankenphp:1-php8.5
+    volumes:
+      - .:/app/public
+      - caddy_data:/data
+      - caddy_config:/config
+    ports:
+      - "80:80"
+      - "443:443"
+      - "443:443/udp"
+
+volumes:
+  caddy_data:
+  caddy_config:
+```
+
+If you need additional PHP extensions, see [Building Custom Docker Image](docker.md#how-to-install-more-php-extensions).
+
+For framework-specific Docker setups, see [Symfony Docker](https://github.com/dunglas/symfony-docker) and [Laravel](laravel.md#docker).
+
+## Step 5: Consider Worker Mode (Optional)
+
+In [classic mode](classic.md), FrankenPHP works like PHP-FPM: each request boots the application from scratch. This is a safe starting point for migration.
+
+For better performance, you can switch to [worker mode](worker.md), which boots your application once and keeps it in memory:
+
+```caddyfile
+example.com {
+    root /var/www/app/public
+    php_server {
+        root /var/www/app/public
+        worker index.php 4
+    }
+}
+```
+
+> [!CAUTION]
+>
+> Worker mode keeps your application in memory between requests. Make sure your code does not rely on global state being reset between requests. Frameworks like [Symfony](worker.md#symfony-runtime), [Laravel](laravel.md#laravel-octane), and [API Platform](https://api-platform.com) have native support for this mode.
+
+## What You Can Remove
+
+After migrating, you no longer need:
+
+- Nginx or Apache
+- PHP-FPM (`php-fpm` service/process)
+- FastCGI configuration
+- Self-managed TLS certificates (Caddy handles them automatically)