"Alternatives" section

[pboling]

Currently the top of the readme has a "Synopsis" section that has something approximating an "Alternatives" section.

I'd like for this to become a real "alternatives" section, as a table with the various alternatives, and why you might use them.

@nov You maintain the most prominent alternatives that I'm aware of. Do you have any interest in creating a list table of them? I feel like you'd know best how to describe them. I've never used your gems, and I've actually barely used this gem outside of developing/maintaining it/*, so I don't feel like I have a great grasp of the alternatives landscape, or how to compare them.

/* That's about to change as I'm launching a tool soon that will use a GitHub/GitLab/CodeBerg OAuth2 login.

[nov]

probably doorkeeper is the most major in OAuth2 server library.
rack-oauth2 is also used as a client library, especially through openid_connect gem.