lb4: rework Testing your application (#479)

Update "Testing your application" to match the content
in "Thinking in LoopBack", remove "Advanced testing".

Add Extending LoopBack 4

Fix Jekyll errors, etc

Update Extending LoopBack

pages/en/lb4/Extending-LoopBack-4.md

@@ -10,7 +10,7 @@ summary:
 
 ## Overview
 
-LoopBack 4 is designed to be highly extensible. For architectural rationale and motivation, see [Crafting LoopBack 4](Crafting-LoopBack-4.html).
+LoopBack 4 is designed to be highly extensible. For architectural rationale and motivation, see [Crafting LoopBack 4](Crafting-LoopBack-4.html). 
 
 ## Building blocks for extensibility
 
@@ -54,7 +54,7 @@ const logger: Logger = await ctx.get('logger');
 logger.log('Hello');
 ```
 
-How does the IoC container have anything to do with extensibility?
+How does the IoC container have anything to do with extensibility? 
 
 1. An alternative implementation of the service provider can be bound the context to replace the existing one. For example, we can implement different hashing functions for password encryption. The user management system can then receive a custom password hashing.
 
@@ -100,11 +100,12 @@ For a list of candidate extensions, see [loopback-next issue #512](https://githu
 
 ### System vs Application extensions
 
-Some extensions are meant to extend the programming model and integration capability of the LoopBack 4 framework.
+Some extensions are meant to extend the programming model and integration capability of the LoopBack 4 framework. 
 
 An application can be decomposed into multiple components too
 
 ## Examples & Tutorials
 
 - loopback4-example-log-extension
 - @loopback/authentication
+