Make the MenuBar overflow example more realistic.

The problem with the current docs sample is that the MenuBar is the direct child of a SplitLayout which directly adjusts its width. This hides the fact that in most cases, you need to give the MenuBar a width relative to its container (e.g. 100%), and also make sure the min-width is set to something that allows it to shrink (e.g. var(--lumo-size-m)).

We need to keep the SplitLayout in the sample to showcase the feature properly, but we should wrap the MenuBar in some other component – e.g. a HorizontalLayout – and provide the width and min-width appropriately. This will help developers figure out how to use the overflow feature.

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Open the MenuBar overflow section linked in the issue and inspect the current SplitLayout sample. Confirm how the sample handles the MenuBar container width and minimum width; it is done when the example uses a wrapper with the stated sizing and still demonstrates overflow correctly.