[BurgerButton] Improvements and fixes

[vasilii-kovalev]

1. The component doesn't apply the user's cx. It seems, because it ignores it in applyBurgerButtonMods. Would be great if it does, because adding it via rawProps.className resets all the component's styles, so we need to wrap it in a div and adjust styles via CSS as div > :global(.uui-button-box) {}
2. The component accepts indentLevel, and even uses it in the applyBurgerButtonMods, but the (S)CSS doesn't change the padding. Looks like a bug
3. The padding is removed when isDropdown is true, which moves the item to the left side of the panel/screen, breaking the alignment with other items (not dropdowns). Looks like a bug
4. Add an example in the documentation with nested structure. Currently, we use a local useState to handle open/close state of expandable items, but maybe you come up with a better solution (or a new component for such dropdowns)

[vasilii-kovalev]

Regarding the point about adding an example with nested structure. I found out about type property, which renders the item with some additional indentation and a vertical line:

Is it how a nested structure should look like? If there are a lot of sub-items and items on the first level, the user would need to scroll down. The implementation we have now renders the UI like this:

A combined version (secondary type for sub-items + foldable parent item):

So I'd expect the new example to show which one is a recommended way.