docs: add troubleshooting guide and update environment examples

- Add TROUBLESHOOTING.md with comprehensive issue resolution guide
- Update .env.example with additional configuration options
- Include solutions for Mixed Content, CSP, and database issues
- Add network, performance, and upgrade troubleshooting sections

Co-Authored-By: Claude Sonnet 4 <noreply@anthropic.com>

.env.example

@@ -8,12 +8,25 @@ CMD_DB_URL=postgres://codimd:CHANGE_THIS_TO_A_STRONG_PASSWORD@database/codimd
 
 # CodiMD Domain Configuration
 # For local development, use localhost
-# For production, set your actual domain (e.g., hackmd.example.com)
+# For production with HTTPS, set your actual domain (e.g., md.automodules.com)
 CMD_DOMAIN=localhost
 
 # URL Configuration
+# Set CMD_PROTOCOL_USESSL=true when using HTTPS (with reverse proxy)
+# Set CMD_URL_ADDPORT=false when using standard ports (80/443)
 CMD_URL_ADDPORT=false
 CMD_PROTOCOL_USESSL=false
 
 # CodiMD Session Secret (generate a random string)
 CMD_SESSION_SECRET=CHANGE_THIS_TO_A_RANDOM_SECRET_STRING
+
+# Optional: Allow iframe embedding (if needed)
+# CMD_ALLOW iframeorigin=https://example.com
+
+# Optional: Session lifetime in milliseconds (default: 1209600000 = 14 days)
+# CMD_SESSION_LIFETIME=1209600000
+
+# Optional: Maximum upload size in bytes (default: 50MB)
+# CMD_IMAGE_UPLOAD_TYPE=filesystem
+# CMD_MAX_UPLOAD_SIZE=52428800
+