doc: rework pareto scoring references

[TomCools]

The documentation seems to equate Pareto Optimization to mult-objective optimization. It explains what it is, that it can be combined with other forms of scoring, only to end with the statement "Pareto scoring is currently not supported in Timefold Solver".

Rework goals:

• Make it clearer up-front we don't support Pareto, but explain our reasoning.
• Make it clear we do support multi-objective optimization, through the use of constraint weights.