Add default HTML page for unconfigured domains

[mrrobot47]

This pull request updates the default server response for the nginx-proxy image to provide a more user-friendly error page when a domain is not configured, instead of returning a generic 503 error. The changes ensure that requests to unconfigured domains serve a custom HTML page with a clear message.

Default error page improvements:

• Added a new default.html file containing a styled message indicating the domain is not configured or disabled. (nginx-proxy/default.html)
• Updated the nginx.tmpl template to serve default.html for requests to unconfigured domains, replacing the previous return 503 directive. (nginx-proxy/nginx.tmpl)
• Changed default server listen directives to use default_server for both HTTP and HTTPS, ensuring the default configuration is properly applied. (nginx-proxy/nginx.tmpl)

Docker image adjustments:

• Modified the Dockerfile to copy the new default.html into the appropriate nginx HTML directory. (nginx-proxy/Dockerfile)