docs: improve Rspack / Rsbuild plugin guide

[chenjiahan]

Description

Improve Rspack / Rsbuild plugin guide:

• Add example for registering the plugin in rspack.config.ts
• Improve the Rsbuild config example, make it easier to understand. For example, use the object format of tools.rspack.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist

• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated the documentation.

[changeset-bot]

⚠️ No Changeset found

Latest commit: c2fdfa7

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