eqn, grap, groff, pic, tbl, troff: add page (#6868)

pages/common/eqn.md

@@ -0,0 +1,13 @@
+# eqn
+
+> Equation preprocessor for the groff (GNU Troff) document formatting system.
+> See also `troff` and `groff`.
+> More information: <https://manned.org/eqn>.
+
+- Process input with equations, saving the output for future typesetting with groff to PostScript:
+
+`eqn {{path/to/input.eqn}} > {{path/to/output.roff}}`
+
+- Typeset an input file with equations to PDF using the [me] macro package:
+
+`eqn -T {{pdf}} {{path/to/input.eqn}} | groff -{{me}} -T {{pdf}} > {{path/to/output.pdf}}`