Add comment about Cors config

Confirm you've already contributed to this project or that you sponsor it

• I confirm I'm a sponsor or a contributor

Describe the solution you'd like

It was quite impossible to figure out how to set a specific cors policy for Openiddict rather than open all endpoints to a permissive default policy.

Can't figure out where it belongs in docs though.

Program.cs:

//BEFORE host.UseCors();
host.Use((context, next) => {
   if (!context.Request.Path.StartsWithSegments("/.well-known/openid-configuration")) return next(context);
   if (context.GetEndpoint() is not null) return next(context);
   context.SetEndpoint(new(null, new(new EnableCorsAttribute("MyCorsPolicyForOpeniddict")), null));
   return next(context);
});
host.UseCors();
host.UseAuthentication();

The other endpoints are map-able, so that can be done via normal procedure. E.g.

app.MapPost("/connect/token", [EnableCorsAttribute("MyCorsPolicyForOpeniddict)] [AllowAnonymous] async (HttpContext      context, ...) => ...);

Additional context

No response

Start by reviewing the Program.cs CORS examples in the issue, including host.UseCors(), the discovery endpoint, and app.MapPost("/connect/token"). Locate the relevant OpenIddict documentation section and add a concise explanation of the specific-policy configuration; done means readers can distinguish the discovery endpoint setup from the normally mapped endpoints.