Improving documentation

Some small issues in the documentation:

• On the HPC tutorial (/tutorial/hpc/mpcdf/), the login node gatezero.mpcdf.mpg seems to be inaccessible
• On the HPC tutorial (/tutorial/hpc/mpcdf/), the link https://docs.mpcdf.mpg.de/faq/2fa.html seems broken because of the last parentesis )
• On the HPC tutorial (/tutorial/hpc/mpcdf/), the link https://docs.mpcdf.mpg.de/doc/computing/software/environment-modules.html seems broken because of the last parentesis )
• On the HPC tutorial (/tutorial/hpc/slurm/), the link https://docs.mpcdf.mpg.de/doc/computing/ seems broken because of the last parentesis )
• I think it's specified in the Readme, but maybe the METIS CMake flag -DCMAKE_DISABLE_FIND_PACKAGE_METIS=On might be explicitly mentioned in the webpage too to avoid confusion (until #1087 is solved): /manual/installation/cmake/ ?
• In tutorial/hpc/gpu/, it says that the only vendor supported is Nvidia

Sorry about the repetitions!