net{perf,server}: add page (#12075)

* netperf: add pages

* move pages to common; adjust syntax

* use placeholder syntax for port numbers

* Add mnemonics and manned.org link per style guide

* missing period

* Add commas

pages/common/netserver.md

@@ -0,0 +1,21 @@
+# netserver
+
+> Server-side command for `netperf`, the benchmarking application that measures network throughput.
+> See also: `netperf`, for the client-side command.
+> More information: <https://manned.org/netserver.1>.
+
+- Start a server on the default port (12865) and fork to background:
+
+`netserver`
+
+- Start server in foreground and do not fork:
+
+`netserver -D`
+
+- Specify [p]ort:
+
+`netserver -p {{port}}`
+
+- Force IPv[4] or IPv[6]:
+
+`netserver -{{4|6}}`