ghdl: add page (#4589)

pages/common/ghdl.md

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+# ghdl
+
+> Open-source simulator for the VHDL language.
+> More information: <http://ghdl.free.fr>.
+
+- Analyze a VHDL source file and produce an object file:
+
+`ghdl -a {{filename.vhdl}}`
+
+- Elaborate a design (where `{{design}}` is the name of a configuration unit, entity unit or architecture unit):
+
+`ghdl -e {{design}}`
+
+- Run an elaborated design:
+
+`ghdl -r {{design}}`
+
+- Run an elaborated design and dump output to a waveform file:
+
+`ghdl -r {{design}} --wave={{output.ghw}}`
+
+- Check the syntax of a VHDL source file:
+
+`ghdl -s {{filename.vhdl}}`
+
+- Display the help page:
+
+`ghdl --help`