Any good way to "document" websites using "Sign in with X" in 1Password?

That has actually been my practice more or less a well. Problem is some services actually only allow sso, no own credentials.

I'm exploring using tags for them now though, since I discovered you can nest tags with forward slash. So for GitHub and services using "sign in with GitHub", I've started tagging them with Auth/GitHub. And similar with other services Auth/Microsoft and Auth/Facebook.

While going through my accounts I also added credentials where I could though, as I find that much "safer". Like, what if the service I sign in with disappears or goes down.

There is no suggested workflow within 1Password.
Because I have the same problem as you (unable to remember if I signed up on some website with sso, or if I signed up with own credentials), I decided to never sign up with sso credentials but always with own credentials.
Why never: I had issues when I tried to break the connection to the sso account and start using own credentials. Some websites were unable to enable me to create an initial password, so I wasn't able to login afterwards, and password reset functions were defunct as well. UPS website was one example.

So I personally recommend you just ignore these "login with xyz" and always sign up with your own data. You have 1Password to store the credentials, and it has essentially unlimited space to store arbitrary numbers of accounts, so there is no benefit in using sso. Sso is for people who cannot remember more accounts, but there is a password manager that will remember it for you.