📚[DOC]: Further explanation and more examples on integral parts and normal_dot_vel needed #144

How would you describe the priority of this documentation request

Critical (currently preventing usage)

Is this for new documentation, or an update to existing docs?

New

Describe the incorrect/future/missing documentation

In the examples we can find the aneurysm example where is considered integral continuity. Although in the documentation nor in the code there is no further explanation of the idea behind that. In the code we can find

`    
# Integral Continuity 2
integral_continuity = IntegralBoundaryConstraint(
    nodes=nodes,
    geometry=integral_mesh,
    outvar={"normal_dot_vel": -2.540},
    batch_size=1,
    integral_batch_size=cfg.batch_size.integral_continuity,
    lambda_weighting={"normal_dot_vel": 0.1},
)
domain.add_constraint(integral_continuity, "integral_continuity_1")

# Integral Continuity 1
integral_continuity = IntegralBoundaryConstraint(
    nodes=nodes,
    geometry=outlet_mesh,
    outvar={"normal_dot_vel": 2.540},
    batch_size=1,
    integral_batch_size=cfg.batch_size.integral_continuity,
    lambda_weighting={"normal_dot_vel": 0.1},
)
domain.add_constraint(integral_continuity, "integral_continuity_1")`

It would be helpful to understand why it is setup in such specific way, using two integral continuity and why are there opposite signs of normal_dot_vel (I assume that one is for inflow and the other is for outflow). As well further explanation of how to physically interpret the normal_dot_vel and how to handle in the code would be appreciated.

If this is a correction, please provide a link to the incorrect documentation. If this is a new documentation request, please link to where you have looked.

https://docs.nvidia.com/deeplearning/modulus/modulus-sym/user_guide/intermediate/adding_stl_files.html