Example/tutorial contribution: Poisson equation with boundary conditions showing how to build meshes and define weak forms

Hi Gridap,

I have been exploring Gridap.jl, and I would like to contribute a small tutorial demonstrating how to set up a Poisson equation with boundary conditions, focusing on mesh creation and test function parameter naming.

Could this be helpful to the examples section?

Start by reviewing the existing examples section and tutorials related to Poisson equations in Gridap.jl. Add a small tutorial covering mesh creation, boundary conditions, weak-form setup, and test-function parameter naming, with the completed example demonstrating the requested workflow.

Written by the indexing model from the issue text.