Update PromptTemplates documentation for customization

Revised the documentation to include detailed instructions on customizing prompt templates, including examples for implementing and configuring custom PromptTemplateSource and TemplateRenderer. Improved clarity by removing default configuration details and adding a section on how the mechanisms work.

docs/PromptTemplates.md

@@ -1,25 +1,13 @@
-# Using Prompt Templates with AiServices
+# Customizing Prompt Templates
 
-This guide demonstrates how to configure and use prompt templates with the AiServices module. This setup involves
-configuring prompt templates, creating prompt template sources, and rendering templates with specific variables.
-Additionally, a test class validates the functionality of these templates.
-
-## Configuration
-
-First, make sure you have the necessary configuration in your `langchain4j-kotlin.properties` file. This is default
-configuration:
-
-```properties
-prompt.template.source=me.kpavlov.langchain4j.kotlin.prompt.ClasspathPromptTemplateSource
-prompt.template.renderer=me.kpavlov.langchain4j.kotlin.prompt.SimpleTemplateRenderer
-```
-
-This configuration specifies the source and renderer for the prompt templates.
+This guide demonstrates how to configure and use prompt templates with
+the [LangChain4J's AiServices](https://docs.langchain4j.dev/tutorials/ai-services). This setup involves
+configuring prompt templates, defining and extending prompt template sources and template rendering.
 
 ## Creating and Using Prompt Template
 
-Include the necessary classes. Your prompt templates will be sourced from the classpath and rendered using a simple
-template renderer.
+Let's start with built-in mechanizm of loading prompt template from classpath. Your prompt templates should be located
+in the classpath, e.g.
 
 File: `prompts/default-system-prompt.mustache`
 
@@ -74,13 +62,14 @@ System and user prompts will be:
 - **System prompt:** "You are helpful assistant using chatMemoryID=default"
 - **User Prompt:** "Hello, My friend! How are you?"
 
-In this example, `TemplateSystemMessageProvider` handles the system prompt template and `AiServices` uses the templates
-to generate responses.
+## How does it work
 
-`PromptTemplateFactory` is registered automatically via Java ServiceLoaders mechanism and is used to provide
-`PromptTemplate`. This class is responsible for obtaining prompt templates from a `PromptTemplateSource` and rendering
-them using a `TemplateRenderer`. If the specified template cannot be found, it will fallback to using the input template
-content.
+In the default implementation, `TemplateSystemMessageProvider` handles the system prompt template and `AiServices` uses
+the templates to generate responses.
+
+`PromptTemplateFactory` provides `PromptTemplateFactory.Template` for `AiServices`. It is registered automatically via
+Java ServiceLoaders mechanism. This class is responsible for obtaining prompt templates from a `PromptTemplateSource`.
+If the specified template cannot be found, it will fallback to using default LC4J's the input template content.
 
 `ClasspathPromptTemplateSource` is implementing `PromptTemplateSource` interface and provides a mechanism to load prompt
 templates from the classpath using the template name as the resource identifier. It attempts to locate the template file
@@ -90,12 +79,89 @@ overridden.
 Implementers of the `TemplateRenderer` interface will typically replace placeholders in the template with corresponding
 values from the variables map.
 
-`SimpleTemplateRenderer` finds and replaces placeholders in the template in the format `{{key}}`, where `key`
+`SimpleTemplateRenderer` finds and replaces placeholders in the template in the Mustache-like format `{{key}}`, where
+`key`
 corresponds to an entry in the variables map. If any placeholders in the template are not defined in the variables map,
 an `IllegalArgumentException` will be thrown.
 
 `RenderablePromptTemplate` implements both `PromptTemplate` and LangChain4j's `PromptTemplateFactory.Template`
 interfaces. It uses a `TemplateRenderer` to render the template content using provided variables.
 
+## Customization
+
+You may customize templates via configuration file `langchain4j-kotlin.properties`, located in the classpath.
+
+| Key                        | Description       | Default Value                                                        |
+|----------------------------|-------------------|----------------------------------------------------------------------|
+| `prompt.template.source`   | Template source   | `me.kpavlov.langchain4j.kotlin.prompt.ClasspathPromptTemplateSource` |
+| `prompt.template.renderer` | Template renderer | `me.kpavlov.langchain4j.kotlin.prompt.SimpleTemplateRenderer`        |
+
+### Extending PromptTemplateSource
+
+To create a custom template source, implement the PromptTemplateSource interface:
+
+```kotlin
+interface PromptTemplateSource {
+  fun getTemplate(name: TemplateName): PromptTemplate?
+}
+```
+
+Example implementation for Redis and Jedis:
+
+```kotlin
+package com.example
+
+// Redis/Jedis-backed template source
+
+class RedisPromptTemplateSource(private val jedis: Jedis) : PromptTemplateSource {
+  override fun getTemplate(name: TemplateName): PromptTemplate? {
+    return jedis.get(name)?.let {
+      SimplePromptTemplate(it)
+    }
+  }
+}
+```
+
+Register your implementation in the `langchain4j-kotlin.properties` configuration file:
+
+```properties
+prompt.template.source=com.example.RedisPromptTemplateSource
+```
+
+### Extending TemplateRenderer
+
+To create a custom template renderer, implement the TemplateRenderer interface:
+
+```kotlin
+interface TemplateRenderer {
+  fun render(
+    template: TemplateContent,
+    variables: Map<String, Any?>
+  ): String
+}
+```
+
+Example implementation:
+
+```kotlin
+package com.example
+
+// Freemarker-based renderer
+class MyTemplateRenderer : TemplateRenderer {
+
+  override fun render(template: TemplateContent, variables: Map<String, Any?>): String {
+    TODO("Add implementation here")
+  }
+}
+```
+
+Register your implementation in the `langchain4j-kotlin.properties` configuration file:
+
+```properties
+prompt.template.renderer=com.example.MyTemplateRenderer
+```
+
+## Examples
+
 You may find the unit test with the
 example [here](../langchain4j-kotlin/src/test/kotlin/me/kpavlov/langchain4j/kotlin/service/ServiceWithPromptTemplatesTest.kt)