stab: Implement MaxWeightStableSetPerturber; update tests and docs

tests/fixtures/gen_stab.py

@@ -7,6 +7,7 @@ from miplearn.collectors.basic import BasicCollector
 from miplearn.io import write_pkl_gz
 from miplearn.problems.stab import (
     MaxWeightStableSetGenerator,
+    MaxWeightStableSetPerturber,
     build_stab_model_gurobipy,
     build_stab_model_pyomo,
 )
@@ -17,9 +18,12 @@ gen = MaxWeightStableSetGenerator(
     w=uniform(10.0, scale=1.0),
     n=randint(low=50, high=51),
     p=uniform(loc=0.5, scale=0.0),
-    fix_graph=True,
 )
-data = gen.generate(3)
+pr = MaxWeightStableSetPerturber(
+    w_jitter=uniform(0.9, scale=0.2),
+)
+base_instance = gen.generate(1)[0]
+data = pr.perturb(base_instance, 3)
 
 params = {"seed": 42, "threads": 1}