rewrite optimization tutorial

[ArnoStrouwen]

Turns the optimization tutorial into an actual Documenter example.

However, I'm stuck at the guess metadata.
OptimizationProblem complains that no guess is present for x.

I also think the comma present after x and y looks a bit weird:

@variables begin
    x, [bounds = (-2.0, 2.0), guess = 1.0]
    y, [bounds = (-1.0, 3.0), guess = 3.0]
end

No such comma is present in the examples on:
https://docs.sciml.ai/ModelingToolkit/stable/basics/Variable_metadata/#Additional-functions
Yet deleting them here gives a whitespace error.

[ChrisRackauckas]

I also think the comma present after x and y looks a bit weird:

It's a detail that comes from Julia's parser.

[ArnoStrouwen]

Apparently, the guess metadata is only compatible with initialization problems for ODE.
I mistakenly thought this would have also transferred to nonlinear problem and optimization problem.
Instead, I had to use the default value.

When this passes tests, it should be good to go.

[ChrisRackauckas]

Yes, guess semantics are for embedded initialization problems. The starting value for an optimization or nonlinear problem is not considered a guess in these semantics, but the initial state.