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.

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)
Closes #19157

See merge request !4979
(cherry picked from commit 48843c0d)