Improve Poisson tutorial with clearer explanations

[ssasmal1759]

This PR expands the Poisson tutorial with additional explanations to help beginners. I added:

• Intro explanation of the weak form and how it connects to a(u,v) and b(v) • Clarification on the mesh file (model.json) and boundary tags • Explanation of test space (V0) and trial space (Ug) • Small transition explanations between sections • Math-to-code comments showing how the weak form maps to Gridap expressions • Clearer visualization guidance to interpret results

Please let me know if anything should be changed, thank you.