Fix broken links to outdated/webpack.md

Two user-facing docs were linking to docs/outdated/webpack.md which is
excluded from the website, causing 404 errors.

Changes:
- docs/core-concepts/webpack-configuration.md: Updated to link to Shakapacker's
  webpack customization docs (most relevant for users customizing webpack)
- docs/deployment/troubleshooting-build-errors.md: Updated to link to our own
  webpack-configuration.md (keeps users in our documentation)

Context:
webpack.md was moved to outdated/ in commit e08f91d (Oct 3, 2025) because
it contained "outdated Webpack v2 content". The file references Webpack v2
patterns and links to a 2017 forum post. These two links were not updated
when the file was moved, causing broken references.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: ihabadham

docs/core-concepts/webpack-configuration.md

@@ -22,7 +22,7 @@ A key decision in your use React on Rails is whether you go with the Shakapacker
 
 ## Option 1: Default Generator Setup: Shakapacker app/javascript
 
-Typical Shakapacker apps have a standard directory structure as documented [here](https://github.com/shakacode/shakapacker/blob/master/README.md#configuration-and-code). If you follow [the basic tutorial](../getting-started/tutorial.md), you will see this pattern in action. In order to customize the Webpack configuration, consult [Webpack Tips](../outdated/webpack.md).
+Typical Shakapacker apps have a standard directory structure as documented [here](https://github.com/shakacode/shakapacker/blob/master/README.md#configuration-and-code). If you follow [the basic tutorial](../getting-started/tutorial.md), you will see this pattern in action. In order to customize the Webpack configuration, consult the [Shakapacker webpack customization docs](https://github.com/shakacode/shakapacker#webpack-configuration).
 
 The _advantage_ of using Shakapacker to configure Webpack is that there is very little code needed to get started, and you don't need to understand really anything about Webpack customization.
 

docs/deployment/troubleshooting-build-errors.md

@@ -276,5 +276,5 @@ fi
 ## Need More Help?
 
 - Check the [general troubleshooting guide](../deployment/troubleshooting-when-using-shakapacker.md)
-- Review [webpack configuration docs](../outdated/webpack.md)
+- Review [webpack configuration guide](../core-concepts/webpack-configuration.md)
 - Contact [[email protected]](mailto:[email protected]) for professional support