Add combineSlices section to Code Splitting page

[EskiMojo14]

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name: :book: New/Updated Documentation Content about: Adding a new docs page, or updating content in an existing docs page

PR Type

Updating existing page

Checklist

• [x] Is there an existing issue for this PR?
  • https://github.com/reduxjs/redux/issues/4662
• [x] Have the files been linted and formatted?

What docs page is being added or updated?

• Section: Usage
• Page: Code Splitting

For Updating Existing Content

What updates should be made to the page?

Now that RTK has its own code-splitting utilities (combineSlices and createDynamicMiddleware), it would be very useful to have an in-depth of how these are intended to be used, separate from the API reference.

Do these updates change any of the assumptions or target audience? If so, how do they change?

This would avoid the standard "recipe" approach in the current docs in favour of depending on an actual library.

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