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)

Marked the task Squashed related commits together as completed

Marked the task Branch has no merge conflicts with master (if you do - rebase it please) as completed

Marked the task Conform by the style guides as completed

Reassigned to @fatihacet

Reassigned to @lbennett

Added 75 commits:

• 73d43c5c...84113d7e - 74 commits from branch master
• 8cdee2ba - Exposed 'onToggleHelp() to window object so showHelp() can be a global function'

Reassigned to @fatihacet

Thank you for updating fat arrow to thin arrow.

Status changed to merged

mentioned in commit 48843c0d

Milestone changed to %8.10

Since this is a regression, can it be merged into next patch release of 8.9.x?

cc @rspeicher

Milestone changed to %8.9

Added ~149423 label

@blackst0ne I agree, this is a regression, so we'll get it in a patch.

mentioned in issue #18343 (closed)

mentioned in issue #19488 (closed)

Removed ~149423 label