Update convergence-related tutorials

https://docs.siesta-project.org/projects/siesta/en/stable/tutorials/basic/scf-convergence/index.html

https://docs.siesta-project.org/projects/siesta/en/stable/tutorials/basic/kpoint-convergence/index.html

https://docs.siesta-project.org/projects/siesta/en/stable/tutorials/basic/grid-convergence/index.html