docs: add custom commands documentation with examples for Java and Kotlin

Author: zachwaffle4

.vitepress/sidebar/guide.mts

@@ -25,6 +25,12 @@ export default [
             {text: "TeleOp", link: "/guide/opmodes/teleop"},
         ]
     },
+    {
+        text: "Commands",
+        items: [
+            {text: "Overview", link: "/guide/commands/custom-commands"},
+        ]
+    },
     {
         text: "Further Reading",
         link: "/guide/further-reading"

src/guide/commands/custom-commands.md

@@ -0,0 +1,51 @@
+# Custom Commands
+
+While NextFTC includes many useful commands, 
+you often will need to create your own.
+
+The recommended way to create a command is by using the `LambdaCommand` class, 
+which allows you to define the command's behavior using lambda functions.
+This approach is concise and flexible, making it easy to create custom commands
+without needing to create a new class for each command.
+
+Here is how the builtin `RunToPosition` command could
+be implemented using `LambdaCommand`:
+
+::: tabs key:code
+
+== Java
+```java
+public static Command runToPosition(ControlSystem system, double position, KineticState tolerance) {
+    return new LambdaCommand("RunToPosition(" + position + ")")
+            .setStart(() -> system.setGoal(new KineticState(position)))
+            .setIsDone(() -> system.isWithinTolerance(tolerance))
+            .requires(system);
+{
+```
+
+Because Java doesn't have default parameters,
+we must make another method to use a default tolerance.
+This example uses a tolerance of 10.0:
+
+```java
+public static Command runToPosition(ControlSystem system, double position) {
+    return runToPosition(system, position, new KineticState(10.0));
+}
+```
+
+== Kotlin
+```kotlin
+fun runToPosition(
+    system: ControlSystem, 
+    position: Double, 
+    tolerance: KineticState = KineticState(10.0)
+) = LambdaCommand("RunToPosition($position)")
+    .setStart { system.goal = KineticState(position) }
+    .setIsDone { system.isWithinTolerance(tolerance) }
+    .requires(system)
+```
+
+:::
+
+The `setStart`, `setUpdate`, `setIsDone`, and `setStop` 
+methods directly map to the methods of the `Command` class.