Hello Dear all contributors,

I hope you're doing well. My name is Joseph Jaramillo, and I am a civil engineer. I have added a new example to the OpenSeesPyDoc project. This example focuses on the nonlinear dynamic analysis of a multi-degree-of-freedom system under an earthquake, and I believe it will be very helpful for users who want to understand how to perform this type of analysis using the repository.

Summary of changes:
I added an example in the nonlinear_mdof.py file that demonstrates how to perform the nonlinear dynamic analysis of a multi-degree-of-freedom system under an earthquake using zeroLength elements and uniaxialMaterial materials. The example includes a simulation of the system's behavior under seismic excitation, providing a practical demonstration of how to set up and analyze this type of problem in OpenSeesPy.
For more details, you can check the following link, which shows the final result of what the page would look like with the implemented example:
NonLinear MDOF: https://jjaramillod93-openseespydoc.readthedocs.io/en/latest/src/NonlinearMDOF.html

Please let me know if you have any questions or need any adjustments to the proposed changes. I'd be happy to collaborate to make this PR as useful as possible.

Thank you for your time and for all the effort you put into this project.

Best regards,
Joseph Jaramillo
Msc. Eng. Joseph Jaramillo - National University of Engineering - PERU