[Dev] Add non zero dim control case with variable

@ocots
- add an example with a control, a variable and *shooting* that uses the transversality condition on $p_v(t_f)$ ($= 0$ if the variable $v$ is not in the cost)
- update the [doc](https://control-toolbox.org/OptimalControl.jl/dev/example-control-free.html#Indirect-method) with $p_v(0) = 0$ ($v(0)$ is free and one can always assume that it is not involved in the cost, using $v(t_f)$ instead); NB. this is why $p_v(0)$ needs not to be passed when evaluating the flow