Sidebar expandable sections toggles accessibility

[afercia]

All the expandable sections in the sidebar:

would need to inform assistive technologies users the sections are actually expandable. I'd propose a few simple changes, following patterns already used in core:

• add an aria-expanded attribute to communicate the section state
• use an aria-label to clarify what the toggle button action will do (different examples of this can be seen, for example, in the Customizer)