[DOCS] Add documention on loads #48
Description
As @mgovers mentioned: The documentation doesn't mention loads a lot.
E.g. the first time "load" is mentioned in the random grid generator example is only in the print statements of the first cell: print(f"Number of loads: {len(grid.sym_load)}") and the quickstart doesn't even mention them at all.
Task
Add documentation on how loads can be used within power-grid-model-ds