update docs, add load_all(), tests for load_all, make through field optional

tests/test_inheritance_concrete.py

@@ -121,11 +121,11 @@ class Bus(Car):
     max_persons: int = ormar.Integer()
 
 
-class PersonsCar(ormar.Model):
-    class Meta:
-        tablename = "cars_x_persons"
-        metadata = metadata
-        database = db
+# class PersonsCar(ormar.Model):
+#     class Meta:
+#         tablename = "cars_x_persons"
+#         metadata = metadata
+#         database = db
 
 
 class Car2(ormar.Model):
@@ -138,7 +138,9 @@ class Car2(ormar.Model):
     name: str = ormar.String(max_length=50)
     owner: Person = ormar.ForeignKey(Person, related_name="owned")
     co_owners: List[Person] = ormar.ManyToMany(
-        Person, through=PersonsCar, related_name="coowned"
+        Person,
+        # through=PersonsCar,
+        related_name="coowned",
     )
     created_date: datetime.datetime = ormar.DateTime(default=datetime.datetime.now)