Write handler API examples with more concise API

[gvdongen]

Use syntax similar to:

const router = restate.keyedRouter({
    greet: async (ctx: restate.RpcContext, name: string) => {
        return `Hello ${name} :-)`;
    },
});

to have easier overview of which routers functions are binded to.

Tasks

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1. Use more concise handler API in documentation #84
   +0
2. Use more concise handler API in hello-world-lambda example #91
   +0
   
3. Use more concise handler API in ticket-reservation API #88
   +0

[gvdongen]

I have done this for some of the examples. Not all of them, since it's sometimes nice to see some different ways of implementing services.