Accessibility: Announce on `<PageTitle />` changes

Created by: umpox

Description

Screen readers do not update when document.title changes. This makes it very difficult for users to be aware that the content of the page has changed, particularly when navigating through a sidebar or a navbar. It seems the best approaches here are:

1. Move focus to next relevant part of page (if suitable)
2. Announce document.title changes to the screen reader through aria-live.

This PR focuses on the latter, it's also a recommended approach by deque (creators of axe).

Related issues (need checking):

• https://github.com/sourcegraph/sourcegraph/issues/34452
• https://github.com/sourcegraph/sourcegraph/issues/34481
• https://github.com/sourcegraph/sourcegraph/issues/35181
• https://github.com/sourcegraph/sourcegraph/issues/35143
• https://github.com/sourcegraph/sourcegraph/issues/35094

Test plan

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.