fix links in markdown files
I'm not sure it is a good idea to keep the link to build.md as it is a part of the source of docs.vtk.org, maybe we should make it link directly to https://docs.vtk.org/en/latest/build_instructions/index.html
I'm not sure it is a good idea to keep the link to build.md as it is a part of the source of docs.vtk.org, maybe we should make it link directly to https://docs.vtk.org/en/latest/build_instructions/index.html