Standardize .env.example files with complete variables and empty values

Changes:
- Reviewed all docker-compose files to identify required variables
- Updated all .env.example files to include ONLY variables used in their respective compose files
- Changed all placeholder values to empty strings for cleaner configuration
- Fixed variable name inconsistency: DOWNLOADS_DIR -> DOWNLOAD_DIR (to match main .env)
- Removed unnecessary variables that aren't used by the compose files

All .env.example files now:
- Contain only the variables actually used in the stack
- Have empty values instead of example placeholders
- Match the variable names used in docker-compose files

This ensures clean, minimal .env files for each stack without any extraneous variables or confusing placeholder values.

docker-compose/core/.env.example

@@ -1,32 +1,26 @@
 # Core Stack Environment Variables
-# These variables are used for Traefik, Authelia, and DuckDNS
+# These variables are used for Traefik, Authelia, DuckDNS, and Pi-hole
 
 # Base Configuration
-DOMAIN=example.duckdns.org
-SERVER_HOSTNAME=myserver
-TZ=America/New_York
+DOMAIN=
+TZ=
+
+# Server Configuration
+SERVER_IP=
+
+# User Configuration
+PUID=
+PGID=
 
 # DuckDNS Configuration
-DUCKDNS_SUBDOMAINS=yourdomain
-DUCKDNS_TOKEN=your-token-here
+DUCKDNS_SUBDOMAINS=
+DUCKDNS_TOKEN=
 
-# Directory Configuration  
-STACKS_DIR=/opt/stacks
+# Pi-hole Configuration
+PIHOLE_PASSWORD=
 
-# Admin Configuration
-DEFAULT_USER=admin
-DEFAULT_PASSWORD=changeme
-DEFAULT_EMAIL=admin@example.com
-
-# Authelia Configuration
+# Authelia Configuration (Generated by script)
 AUTHELIA_JWT_SECRET=
 AUTHELIA_SESSION_SECRET=
 AUTHELIA_STORAGE_ENCRYPTION_KEY=
 AUTHELIA_ADMIN_PASSWORD_HASH=
-
-# SMTP Configuration (Optional)
-SMTP_HOST=smtp.gmail.com
-SMTP_PORT=587
-SMTP_USERNAME=
-SMTP_PASSWORD=
-SMTP_SENDER=noreply@example.com