doc(marathon): add path constrains and algo ideas

Marathon.md

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+# Marathon open problem
+
+We are given a graph `G` and a starting node `S` and must find a path of marathon length (42.195 km)
+with an end node close to `S`.
+
+## Constraints
+
+Constraints for an acceptable path :
+- Path is valid
+- Path is accessible by foot
+- Path nodes are not visited twice
+- Path length is 42 195 +/- 40 meters
+- Starting node is less than 200 m away from end node.
+
+Constraints for an optimal path :
+- Path is acceptable
+- Path length is as close as possible to 42 195 meters
+  AND/OR start and end node are as close as possible
+
+Note : the [Toulouse 2024 marathon][toulouse-2024] start and end points are roughly 200 meters apart.
+
+The algorithm should fail if no acceptable path can be found.
+
+[toulouse-2024]: https://toulouserunexperience.fr/marathon/
+
+## Algo ideas
+
+Use A* with an heuristic taking into account :
+- Path Length
+- and distance to end node.
+
+Score : `42 195 + | 42 195 - pathLength | + distance end to node`.
+