[Discover] Add focus to h1 element when client side routing is executed

[kertal]

When we change the route in Discover, the h1 element of the target route should get focus because of a11y.

This seems correlated with https://github.com/elastic/kibana/issues/42379. I'm in favor of setting focus on the H1 of a page view when we swap URLs using client-side routing, so screen readers announce the change properly. I'll do some more research, but I think these are related items.

Originally posted by @1Copenut in #61870 (comment)

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)

[elasticmachine]

Pinging @elastic/kibana-accessibility (Project:Accessibility)

[1Copenut]

I've discussed this strategy, setting focus on <h1> on route change with @zuhairmahd. We agreed it is a good approach for Kibana services. With multiple teams and applications running, it will take some time, but that's alright. We will point other teams to Discover's work for implementing focus on route change, until all applications have adopted the unified UX pattern.

[kertal]

@1Copenut is more than this to take care of to resolve #42379 ? thx

[1Copenut]

@kertal I think so yes. The EUI team discussed this today in our meeting and we have a plan for getting #42379 finished.

1. @constancecchen is updating the EuiSkipLink component
2. EUI team is exploring a new approach to announce page load on SPA route changes using a live region
3. When this approach is finalized, we will follow up with the Discover team as early adopters and create a meta issue for all product teams to implement the change as it fits their roadmaps.