Add combineSlices section to Code Splitting page

[Ben Durrant]

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.