add(build): include man page in installation build step

build.zig

@@ -23,6 +23,8 @@ pub fn build(b: *std.Build) void {
         .target = target,
         .optimize = optimize,
     });
+    // install man pages along executable
+    b.installFile("doc/sp.1", "man/man1/sp.1");
 
     // This declares intent for the executable to be installed into the
     // standard location when the user invokes the "install" step (the default