I’m creating a website using HTML for my friends, and I decided that logging in with Discord would be the easiest way to have a login system as we all use it. I read the documentation for Discord OAuth2, yet I can’t understand how I would go about implementing a Discord login to a website made with HTML. I would prefer if I didn’t have to use PHP, as I know almost nothing about it.
It's really not possible to implement a login functionality without using a programming language. You would need a way to make API calls to discord so that it can verify the user but this isn't possible with a plain HTML and CSS website.
I would suggest you learn a language like Javascript which will enable you to efficiently interact with the Discord API