[Accessibility] Announce Kibana route changes within an app

[vadimkibana]

• Create a solution which would correctly announce deep route changes within a Kibana application. Potentially how this could work with:
  • Through some integration into the default Kibana application router.
  • Maybe somehow through React components.
• Make use of this new mechanism in some places in Kibana.
• Write up a document of best practices of using this.

Partially addresses #135760

[1Copenut]

The EUI docs site is a good model for how to implement this improvement using EUI components. Having the EuiScreenReaderOnly as the first element in the React root gives us a consistent place to set focus programmatically and sets us up nicely to add the skip to content link as the first user-focusable element.