Solves #351 - Add kotlin docs (#353)

* Solves #351 - Add kotlin docs

* Fix doc typos

docs/index.md

@@ -59,7 +59,7 @@ jte gives you the same productive, typesafe experience you're used to from writi
 
 ## 5 minutes example
 
-Here is a small jte template `example.jte`:
+Here is a small jte template `example.jte` (or `example.kte` if using Kotlin):
 
 === "Java"
 

docs/kotlin.md

@@ -0,0 +1,96 @@
+---
+title: jte Kotlin tips
+description: Details specific to Kotlin usage
+---
+
+# Kotlin Details
+
+Kotlin entails a few differences. The `jte-kotlin` dependency is needed to enable `.kte` templates that contain 
+Kotlin code. Keep in mind that Kotlin application code can use either Java (`.jte`) or Kotlin (`.kte`) templates. `.kte` 
+files are only needed for Kotlin based templates. 
+
+## Add required dependencies
+
+This dependency is a compile time dependency to avoid including the Kotlin compiler in the resulting build.
+
+=== "Maven"
+
+    ```xml linenums="1"
+    <dependency>
+        <groupId>gg.jte</groupId>
+        <artifactId>jte-kotlin</artifactId>
+        <version>{{ latest-git-tag }}</version>
+        <scope>compile</scope>
+    </dependency>
+    ```
+
+=== "Gradle"
+
+    ```groovy linenums="1"
+    compileOnly "gg.jte:jte-kotlin:{{ latest-git-tag }}"
+    ```
+
+## Common errors
+
+### Non-null parameters
+
+=== "Non-null (`.kte file`)"
+
+    ```html linenums="1"
+    @param name: String
+    ```
+
+=== "Nullable (`.kte file`)"
+
+    ```html linenums="1"
+    @param name: String?
+    ```
+
+If a non-null parameter exists in the template, the template will fail to load with an error in the line where the 
+parameter is being included. 
+
+```
+Error rendering template:
+
+Failed to render admin/index.kte, error at admin/index.kte:1
+(many many lines of stack trace)
+Caused by: java.lang.NullPointerException: null cannot be cast to non-null type kotlin.String
+```
+### Writing a non-supported type to a template
+
+```html linenums="1"
+@param myList: List
+
+${myList}
+```
+
+This will generate an error like so:
+
+```
+gg.jte.TemplateException: Failed to compile template, error at myPage/index.kte:3
+			jteOutput.writeUserContent(things)
+/Users/aUser/app/jte-classes/gg/jte/generated/ondemand/index/JteindexGenerated.kt:13:14
+Reason: None of the following functions can be called with the arguments supplied: 
+public open fun writeUserContent(p0: Content!): Unit defined in gg.jte.html.HtmlTemplateOutput
+public open fun writeUserContent(p0: Boolean): Unit defined in gg.jte.html.HtmlTemplateOutput
+(etc.)
+```
+
+This will be improved in the future.
+
+### Nulls in `@for` expressions
+
+`@for` loops use an iterator. If a variable is null then the following error will appear:
+
+```
+Failed to compile template, error at myApp/index.kte:5
+		for (s in things) {
+/Users/aUser/code/cds/jte-classes/gg/jte/generated/ondemand/index/JteindexGenerated.kt:10:13
+Reason: Not nullable value required to call an 'iterator()' method on for-loop range
+```
+
+A quick fix is to make the parameter have an empty list by default.
+
+```html linenums="1"
+@for(things in (things ?: listOf())) 
+```

mkdocs.yml

@@ -85,6 +85,7 @@ nav:
     - "Precompiling Templates": pre-compiling.md
     - "Binary Rendering": binary-rendering.md
     - "Localization": localization.md
+    - "Kotlin Templates": kotlin.md
   - "Extensions":
     - "jte-models Facade Generator": jte-models.md
     - "Extensions API": jte-extension-api.md