[IMPROVEMENT] Update performance guide with scalability

In https://github.com/orgs/PowerGridModel/discussions/24 there is a nice overview of the scalability of Power Grid Model. 
To make this information easier to find, let's extend the [performance guide ](https://github.com/PowerGridModel/power-grid-model/blob/main/docs/user_manual/performance-guide.md) with this information about scalability.
