Add lexicographic Pareto front selection example

[disnexide]

This PR adds a small documentation example for selecting one final solution from an already obtained Pareto front using lexicographic ordering.

The example is intended as a simple decision-making helper: objectives are sorted by priority, and solutions are compared according to this priority order. This can be useful when a multi-objective optimization algorithm returns a set of Pareto-optimal solutions, but one final compromise solution is needed.

All objectives in the example are assumed to be minimized. For maximization objectives, users can multiply the corresponding objective values by -1 before applying the function.