Fix broken links

Author: samdark

src/guide/index.md

@@ -147,5 +147,5 @@ We release this guide under the [Terms of Yii Documentation](https://www.yiifram
 
 ## Extras
 
-- [Cookbook](../../cookbook/en/README.md)
+- [Cookbook](../cookbook/index.md)
 - [Glossary](glossary.md)

src/guide/intro/upgrade-from-v2.md

@@ -1,6 +1,6 @@
 # Upgrading from Version 2.0
 
-> If you haven't used Yii2, you can skip this section and get directly to "[getting started](../start/installation.md)"
+> If you haven't used Yii2, you can skip this section and get directly to "[getting started](../start/prerequisites.md)"
 > section.
 
 While sharing some common ideas and values, Yii 3 is conceptually different from Yii 2. There is no easy upgrade

src/guide/runtime/logging.md

@@ -11,7 +11,7 @@ Using the Yii logging framework involves the following steps:
 
 * Record [log messages](#log-messages) at various places in your code;
 * Configure [log targets](#log-targets) in the application configuration to filter and export log messages;
-* Examine the filtered logged messages exported by different targets (e.g. the [Yii debugger](../tool/debugger.md)).
+* Examine the filtered logged messages exported by different targets (e.g. the Yii debugger).
 
 In this section, the focus in on the first two steps.
 

src/guide/security/best-practices.md

@@ -28,7 +28,7 @@ if (!in_array($sortBy, ['title', 'created_at', 'status'])) {
 }
 ```
 
-In Yii, most probably you'll use [form validation](../input/validation.md) to do similar checks.
+In Yii, most probably you'll use form validation to do similar checks.
 
 Further reading on the topic: