They are just static files so you can use the static site feature of Node-RED, have a look through the comments in your settings.js file in your userDir folder (generally ~/.node-red).
Unfortunetly, my knowledge is so poor, that I would like to ask you for more details about what I should do. I looked through that file and I don't know which lines I should uncomment and what kind of I should add.
You set it to a folder name and then you can put anything into that folder and load it as a web resource. For example, put a file called my.html in there with a basic web page and then navigate to http://<node-red-server-address>:1880/my.html.