[web] Updating link to msh2vtu
Link to msh2vtu provided in Tools/First-Steps points to unmaintained github repo. This MR replaces it with link to msh2vtu page in ogstools userguide.
Link to msh2vtu provided in Tools/First-Steps points to unmaintained github repo. This MR replaces it with link to msh2vtu page in ogstools userguide.