The canonical reference materials for groff are cstr54 (a downloadable PostScript copy of which is available here) and the troff and groff_diff manpages. The most complete and up-to-date source of information is the groff info pages, available by typing info groff at the command line (assuming you have the TeXinfo standalone browser installed on your system, which is standard for most GNU/Linux distributions). And for inputting special characters, see man groff_char.
MOM
$ info groff # most complete and up-to-date information
$ man troff # troff
$ man groff_diff # GNU troff vs classival troff
$ man groff_mom # mom macros, roff language which is part of groff
$ man groff_char # groff glyph names
$ for size in 8 9 10 12 17; do for type in r i b; do sudo install-font.sh -s -F cm -f +${type^^} cm${type}${size}.pfb ; done ; done
fontforge not installed. Aborting.
...
$ sudo pacman -S fontforge