Implement lowering 'decnet'

* src/pf/expand.lua: Implement lowering of 'decnet'.
* src/pf/parse.lua: Add support for parsing 'decnet'.
* doc/Makefile: Add examples.
* doc/decnet-host-10.15.md: Add example of 'decnet host 10.15'.
* doc/decnet-src-10.15.md: Add example of 'decnet src 10.15'.