Add default backend page for unmatched domains

- Add default backend template (hap_default_backend.tpl)
- Add customizable default page HTML template (default_page.html)
- Modify generate_config() to include default backend for unmatched domains
- Add environment variables for customizing default page content:
  - HAPROXY_DEFAULT_PAGE_TITLE
  - HAPROXY_DEFAULT_MAIN_MESSAGE
  - HAPROXY_DEFAULT_SECONDARY_MESSAGE
- Update README with documentation and examples
- Ensure backward compatibility with existing configurations
- Remove email contact link as requested

templates/hap_default_backend.tpl

@@ -0,0 +1,12 @@
+# Default backend for unmatched domains
+backend default-backend
+    mode http
+    option http-server-close
+    http-request set-header X-Forwarded-Proto https if { ssl_fc }
+    http-request set-header X-Forwarded-Port %[dst_port]
+    http-request set-header X-Forwarded-For %[src]
+    http-request set-header X-Real-IP %[src]
+    
+    # Serve the default page HTML response
+    http-response set-header Content-Type text/html
+    http-response set-body "{{ default_page_content }}"