Add xmldocs

Hi and thanks for this project!

It would be nice to add xmldocs on public APIs.

For example I just made a mistake and used `AddElsaCore` instead of `AddElsa`.
They both have the exact same signature, so it compiles. And their names are very similar so it's very easy to make this mistake.

At first I got an error saying IJavaScriptService could not be resolved, so I just added `services.AddJavaScriptExpressionEvaluator();`.

But then obviously nothing was running because AddElsaCore doesn't register the IHostedService.

So I wasted quite a bit of time on this. Some xmldocs could have maybe explained the difference between AddElsaCore and AddElsa.