process(3lua): init man pages

Initialise man pages for the process module.

man/process/process.pid.3lua.scd

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+PROCESS.PID(3lua) "3lua" "Callisto manual pages"
+
+# NAME
+*process.pid* - get the current process's PID
+
+# SYNOPSIS
+*process.pid*()
+
+# DESCRIPTION
+*process.pid* returns the PID of the current interpreter process.
+
+# EXAMPLES
+Create a function that returns a unique name for a temporary file:
+
+```
+function tmpname(prefix)
+	return "/tmp/" .. prefix .. '.' .. process.pid()
+end
+```
+
+# SEE ALSO
+callisto(3lua), process(3lua), getpid(2)