docs: link runnable api (#21783)

docs/docs/how_to/index.mdx

@@ -7,9 +7,9 @@ sidebar_class_name: hidden
 
 Here you’ll find answers to “How do I….?” types of questions.
 These guides are *goal-oriented* and *concrete*; they're meant to help you complete a specific task.
-For conceptual explanations see [Conceptual Guides](/docs/concepts/).
+For conceptual explanations see the [Conceptual guide](/docs/concepts/).
 For end-to-end walkthroughs see [Tutorials](/docs/tutorials).
-For comprehensive descriptions of every class and function see [API Reference](https://api.python.langchain.com/en/latest/).
+For comprehensive descriptions of every class and function see the [API Reference](https://api.python.langchain.com/en/latest/).
 
 ## Installation
 
@@ -26,7 +26,7 @@ This highlights functionality that is core to using LangChain.
 
 ## LangChain Expression Language (LCEL)
 
-LangChain Expression Language is a way to create arbitrary custom chains. It is built on the Runnable protocol.
+LangChain Expression Language is a way to create arbitrary custom chains. It is built on the [Runnable](https://api.python.langchain.com/en/latest/runnables/langchain_core.runnables.base.Runnable.html) protocol.
 
 - [How to: chain runnables](/docs/how_to/sequence)
 - [How to: stream runnables](/docs/how_to/streaming)