Add token handling example for a Resource Server

[brunokc]

The docs do a good job showing how to implement a Client or an Authorization Server using Authlib. I would like to use Authlib to validate tokens from a Resource Server perspective, meaning I'll receive access tokens from a Client that I'll need to ensure are valid and are from one of the Authorization Servers in my "approved" list.

It would be great if the docs and/or the samples could address some Resource Server scenarios.