/usr/share/doc/gromacs/html/user-guide/ is in gromacs-data 2018.1-1.
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/usr/share/doc/gromacs/html/user-guide/cmdline.html
/usr/share/doc/gromacs/html/user-guide/cutoff-schemes.html
/usr/share/doc/gromacs/html/user-guide/environment-variables.html
/usr/share/doc/gromacs/html/user-guide/faq.html
/usr/share/doc/gromacs/html/user-guide/file-formats.html
/usr/share/doc/gromacs/html/user-guide/floating-point.html
/usr/share/doc/gromacs/html/user-guide/flow.html
/usr/share/doc/gromacs/html/user-guide/force-fields.html
/usr/share/doc/gromacs/html/user-guide/getting-started.html
/usr/share/doc/gromacs/html/user-guide/index.html
/usr/share/doc/gromacs/html/user-guide/managing-simulations.html
/usr/share/doc/gromacs/html/user-guide/mdp-options.html
/usr/share/doc/gromacs/html/user-guide/mdrun-features.html
/usr/share/doc/gromacs/html/user-guide/mdrun-performance.html
/usr/share/doc/gromacs/html/user-guide/run-time-errors.html
/usr/share/doc/gromacs/html/user-guide/system-preparation.html
/usr/share/doc/gromacs/html/user-guide/terminology.html