Docs: What are the difference and pros/cons over the new extends feature and the existing merge tools (webpack-merge)

[luzfcb]

Hello, I recently noticed that support for extends was introduced in Rspack 1.3.x.

Is the extends feature intended to replace the use of webpack-merge?
Does it offer the same behavior as the merge function provided by webpack-merge?

I ask because the Rspack configuration documentation still references webpack-merge, while the extends documentation https://rspack.dev/config/#extending-configurations and https://rspack.dev/config/extends is somewhat vague about when and why one should be used over the other.

It would be helpful if the documentation clarified whether extends is a drop-in replacement, when it should be preferred, and if there are any limitations or compatibility concerns.
Thank you!

[chenjiahan]

Both extends and webpack-merge can be used to merge Rspack configuration and their merge results should be basically the same, the main differences are:

• extends is a built-in syntax sugar and is only available when using Rspack CLI.
• webpack-merge requires an additional npm dependency and can be used in any scenario, such as when using Rspack JS API or other Rspack-based tools.

I'll update https://rspack.dev/config/#merge-configurations to add these differences.

[chenjiahan]

Documentation updated in #10159