Enhance menu bar documentation on icons and variants

[rodolforfq]

Added guidance on using MenuBarVariant for drop-down indicators and icon-only menu items. Based con customer's feedback.

[CLAassistant]

All committers have signed the CLA.

[github-actions]

AI Language Review

The only notable change is the addition of text mentioning the use of MenuBarVariant.LUMO_DROPDOWN_INDICATORS and MenuBarVariant.LUMO_ICON for styling menu items. There are no issues requiring improvement in the updated version.

[jouni]

I’m not sure I follow the logic implied by the additional sentences. In what cases should drop-down indicators be used? And I don't think the LUMO_ICON variant hides the drop-down indicators.

Also, as the guidance here is Lumo-specific and doesn't apply to base styles or Aura at all, I would like to avoid theme-specific guidance in the main component docs page. Theme-specific things would be more suitable on the Styling sub-page.

Based con customer's feedback.

What was the feedback?