Resolve "Create a tab shortcut to jump directly to content"

What does this MR do?

Adds accessibility shortcut to page, i.e. if user hits tab key right after the page is loaded and user has not interacted to any part of the page, the shortcut appears at top left corner of the page. Then, pressing enter (spacebar support is WIP) scrolls page to focus on content.

Are there points in the code the reviewer needs to double check?

See if id attribute added to .content element (present in _page.html.haml) needs to be reviewed if it is correct approach or we already have an ID that shortcut can refer to.

Why was this MR needed?

This feature is particularly useful for screen readers where user doesn't have to go through entire nav to reach content of page.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #24057 (closed)