Document coordinate systems (#83)

The game has two different coordinate systems. Each entity has a
position that is described by a point. And then there is the routing
grid, which airplanes follow on their way to the airport. Both have been
added to the documentation, with screenshots visualizing their
properties.

docs/docs/coordinates.md

@@ -0,0 +1,47 @@
+# Coordinates
+
+The games uses two different coordinate systems for two different purposes:
+
+- The _position_ of an airplane on the map is represented by a
+  [`Point`](/docs/api/types#point).
+- The flight plan of an airplane is a list of [`Nodes`](/docs/api/types#node) on
+  a routing grid.
+
+Both systems are described in more detail below.
+
+## Coordinate System
+
+The _coordinate system_ is used to describe the position of entities on the map.
+It uses the [`Point`](/docs/api/types#point) type, which has an `x` and `y`
+field. Every pixel in the game can be addressed with a `Point`.
+
+The origin of the coordinate system is at the center of the map. The x-axis goes
+from left to right, and the y-axis goes from the bottom to the top.
+
+![coordinate system](/img/coordinate-system.png)
+
+## Routing Grid
+
+Airplanes are routed along the _routing grid_. The grid consists of
+[`Nodes`](/docs/api/types#node) that are distributed in an even pattern across
+the map. The following screenshots highlights the nodes that form the routing
+grid in red.
+
+![routing grid](/img/routing-grid.png)
+
+The routing grid is part of the [`Map`](/docs/api/types#map), and can be queried
+through the [`MapService`](/docs/api/Services/map-service).
+
+Every [`Nodes`](/docs/api/types#node) has a field called `restricted` which
+indicates whether an airplane can pass through it. In the screenshot above, the
+nodes around the airport are `restricted`, and are thus not rendered on the map.
+
+Airplanes fly from node to node. While in flight, their position is described by
+a [`Point`](/docs/api/types#point). But their fligh plan, i.e. their _route_, is
+represented by a list of [`Nodes`](/docs/api/types#node).
+
+Since the routing grid overlays the map, the position of every
+[`Node`](/docs/api/types#node) can be converted to a
+[`Point`](/docs/api/types#point) in the [coordinate system](#coordinate-system).
+See the documentation of the [`MapService`](/docs/api/Services/map-service) for
+more information.

docs/sidebars.js

@@ -13,7 +13,7 @@
 
 /** @type {import('@docusaurus/plugin-content-docs').SidebarsConfig} */
 const sidebars = {
-  docs: ["index", "rules"],
+  docs: ["index", "rules", "coordinates"],
   api: [
     {
       type: "autogenerated",