add man pages for shell commands and implement mkdir, ping, matrix, and about commands

src/shell/commands/cat.ts

@@ -12,4 +12,8 @@ export class cat implements IShellCommand {
         if (args.length === 0) return "cat: missing file operand";
         return this.syscall.readFile(args[0]);
     }
+
+    getManPage(): string {
+        return "cat - concatenate files and print on the standard output";
+    }
 }