Wrong transmission conditions in Tutorial 11

@oriolcg (I think you did this test)

I was looking at [Tutorial 11](https://gridap.github.io/Tutorials/dev/pages/t011_fsi_tutorial/#Tutorial-11:-Fluid-Structure-Interaction-1).

You consider a steady FSI problem where `us` is the solid velocity. I think it does not make sense, because the solid velocity should be zero.

I think that we want to solve the solid problem using the solid displacement `ds`. In fact, the constitute model you are using makes sense for the displacement, not the velocity.

Then, we want to impose `uf = 0` on the interface.

That would solve the problem, but then the problem is only coupled in one direction. The matrix is upper triangular, since $A_u^{f,s}$ is zero now. It is ok, I would just clarify in the text.

I wanted to agree on this, before starting to make changes.

Start with Tutorial 11 and review the steady fluid-structure interaction formulation described in the issue, focusing on the solid displacement or velocity and the interface condition. Confirm the intended transmission conditions and coupling direction with the existing tutorial context. Done means the tutorial's formulation and explanatory text consistently describe the agreed model.

Escrito por el modelo de indexación a partir del texto del issue.