Add button to add custom certificate in empty certificate list

[badkeyy]

Issue:

Fixes https://github.com/NginxProxyManager/nginx-proxy-manager/issues/4312

Description:

This PR improves the SSL Certificates list by introducing a new button that allows users to upload a custom SSL Certificate directly. This eliminates the need to navigate through the dropdown menu in the top-right corner of the list, reducing confusion—especially when the list is empty and it appears that only Let's Encrypt Certificates can be added.

Screenshots

[nginxproxymanagerci[bot]]

Docker Image for build 4 is available on DockerHub as nginxproxymanager/nginx-proxy-manager-dev:pr-4311

Note: ensure you backup your NPM instance before testing this image! Especially if there are database changes Note: this is a different docker image namespace than the official image