[Accessibility improvement] Improve breadcrumb menu

[marcus-herrmann]

[This is a ticket for the "accessibility backlog"]

The semantics of the breadcrumb menu could be improved

• By making the unordered list (<ul>) an ordered list (<ol>)
• By wrapping the list into a nav landmark region and give this region an accessible name, to help distinguish this nav region from the other (the main menu) like this: <nav aria-label="Breadcrumb">. Please note that the word navigation should not be part of the accessible name, the screen reader will annouce that this very region is a navigation region
• Add Schema.org metadata: The <ol> should receive the attributes itemscope itemtype="https://schema.org/BreadcrumbList", whereas the single listitems should receive itemtype="https://schema.org/Thing" itemprop="item"
• Set active link to aria-current="true"