RenderableTravelSpeed docs update and more examples (#3885)

* Update docs to clairify that the travel is synced to the simulation time

* Add a couple more examples for RenderableTravelSpeed

* Fix broken example and add description of the things showed

It's not as apparent for the speed indicators, as they will not naturally be shown when focusing

* Fix typo

Clarify that simulation delta time is controlling the speed

* Apply suggestions from code review

Co-authored-by: Alexander Bock <alexander.bock@liu.se>

* Apply suggestions from code review

* Make comments in examples more consistent

---------

Co-authored-by: Alexander Bock <alexander.bock@liu.se>

data/assets/examples/renderable/renderabletravelspeed/travelspeed_appearance.asset

@@ -0,0 +1,36 @@
+-- Indicator Length and Width
+-- This example creates a light speed indicator where the length and width of the line
+-- have been adjusted for the indicator to be more visible across the large distance from
+-- Earth to Mars. The line will move according to the speed of light.
+
+local earthAsset = asset.require("scene/solarsystem/planets/earth/earth")
+local marsAsset = asset.require("scene/solarsystem/planets/mars/mars")
+
+local Node = {
+  Identifier = "RenderableTravelSpeed_Example_LengthAndWidth",
+  Parent = earthAsset.Earth.Identifier,
+  Renderable = {
+    Type = "RenderableTravelSpeed",
+    Target = marsAsset.Mars.Identifier,
+    -- Length in seconds, where 1 second = distance light travels in 1 second
+    IndicatorLength = 60.0,
+    -- Increase line width for visibility
+    LineWidth = 5.0
+  },
+   GUI = {
+    Name = "RenderableTravelSpeed - Indicator Length and Width",
+    Path = "/Examples",
+    Description = [[
+      A light speed indicator from Earth to Mars. The indicator's length corresponds to
+      60 seconds of travel at the speed of light.
+    ]]
+  }
+}
+
+asset.onInitialize(function()
+  openspace.addSceneGraphNode(Node)
+end)
+
+asset.onDeinitialize(function()
+  openspace.removeSceneGraphNode(Node)
+end)

data/assets/examples/renderable/renderabletravelspeed/travelspeed_travelspeed.asset

@@ -0,0 +1,40 @@
+-- Custom Travel Speed
+-- This example creates a speed indicator; a line that moves with a given speed from the
+-- parent node (Earth) to the target node (Jupiter). The travel speed is here set to 1
+-- light-hour per second.
+
+local earthAsset = asset.require("scene/solarsystem/planets/earth/earth")
+local jupiterAsset = asset.require("scene/solarsystem/planets/jupiter/jupiter")
+
+local Node = {
+  Identifier = "RenderableTravelSpeed_Example_TravelSpeed",
+  Parent = earthAsset.Earth.Identifier,
+  Renderable = {
+    Type = "RenderableTravelSpeed",
+    Target = jupiterAsset.Jupiter.Identifier,
+    TravelSpeed = 1.07925285e9, -- 1 light-hour in meters
+
+    -- Length in seconds, where 1 second = distance traveled in 1 second at the set
+    -- travel speed
+    IndicatorLength = 60,
+    -- Increase line width for visibility
+    LineWidth = 5.0
+
+  },
+   GUI = {
+    Name = "RenderableTravelSpeed - Light-hour Speed Indicator",
+    Path = "/Examples",
+    Description = [[
+      A speed indicator from Earth to Jupiter, moving at 1 light-hour per second. The
+      indicator's length corresponds to 60 seconds of travel at the set travel speed.
+    ]]
+  }
+}
+
+asset.onInitialize(function()
+  openspace.addSceneGraphNode(Node)
+end)
+
+asset.onDeinitialize(function()
+  openspace.removeSceneGraphNode(Node)
+end)

modules/space/rendering/renderabletravelspeed.cpp

@@ -105,6 +105,10 @@ namespace {
     // to show more information related to the distance traveled. For example, a length
     // of 1 shows how far an object would move over a duration of one second based on the
     // selected speed.
+    //
+    // The indicator will move from the parent to the target node based on the currently
+    // set simulation time and the set `TravelSpeed`. That is, increasing the simulation
+    // speed makes the indicator move faster.
     struct [[codegen::Dictionary(RenderableTravelSpeed)]] Parameters {
         // [[codegen::verbatim(TargetInfo.description)]]
         std::string target [[codegen::identifier()]];