add structural dynamics example

[vflores-io]

This PR adds an example of the Augmented Kalman Filter for joint input-state estimation in structural dynamics using RxInfer.jl. The example includes:

• A structural toy model setup.
• Synthetic data generation.
• Joint input-state estimation using the AKF smoother.
• The example is based on the blog https://vflores-io.github.io/.

[bvdmitri]

Thanks @vflores-io for the example! Amazing! The core team is at the holiday mood right now, but we are going to review your work as soon as we can. Happy holidays!

[vflores-io]

Thank you @bvdmitri ! Heads-up: I will update in the next commit the shear_model png because in the previous version I forgot to draw the k coefficients.
Happy holidays!