Fixed 'use shortcuts' button on docs

What does this MR do?

Exposes 'onToggleHelp() to window object through showHelp() so a help panel can be toggled globally using showHelp().

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

Is this the best implementation? I actually think this is tidier than doing something like onclick="new Shortcuts().onToggleHelp" or $.trigger 'keydown', char: '?' but let me know.

Why was this MR needed?

Docs UX

What are the relevant issue numbers?

Closes #19157 (closed).

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 style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together Closes #19157 (closed)