Add graph illustrating the "finish route" buttons

Based on https://forum.minetest.net/viewtopic.php?p=404223#p404223

assets/manual/manual.tex

@@ -3,12 +3,13 @@
   \def\pgfsysdriver{pgfsys-dvisvgm4ht.def} % for HTML output
 \fi
 \usepackage[a4paper,margin=2cm]{geometry}
+\usepackage[T1]{fontenc}
 \ifx\HCode\undefined
 \usepackage{beton}
 \usepackage{eulervm}
 \fi
-\usepackage[T1]{fontenc}
 \setlength{\parindent}{0pt}
+\setlength{\parskip}{1ex}
 \usepackage{listings}
 \usepackage{hyperref}
 \usepackage{amsmath,amssymb,siunitx}
@@ -668,6 +669,19 @@ You can also click ``Finish route at the end of NEXT section'' at the starting T
 
 If the TCB is not considered suitable for route continuation, please check that the side of the TCB from which the train passes the TCB is part of a track section and that the train can reach there without passing a point assigned to another TCB. If you see ``Advancing over next section is impossible at this place. End of interlocking.'', please check that the side of the TCB corresponding to the driving direction is part of a track section.
 
+\begin{centeredtikzpicture}
+  \draw [->] (-0.5,1) node [left] {Finish route HERE} -- (4.5,1);
+  \draw [->,blue,very thick] (0,1) -- (2,1);
+  \draw [->] (-0.5,0) node [left] {Finish route at the end of NEXT section}-- (4.5,0);
+  \draw [->,blue,very thick] (0,0) -- (4,0);
+  \foreach \y in {1,0} {
+    \draw (0,\y) pic [xscale=-1] {signal=red} node [below] {$P$};
+    \foreach \x/\l in {2/Q,4/R} {
+      \draw (\x,\y) pic {tcb} node [below] {$\l$};
+    }
+  }
+\end{centeredtikzpicture}
+
 After finishing the route, you are prompted for the name of the route. It is recommended to use a sensible name. You should then see the route formspec. If you don't, click on the name of the route you just created and click ``Edit route'', and proceed to the next section regarding ARS.
 
 \subsection{Automatic routesetting}\label{s:ilars}