Add documentation for keyboard shortcuts for expand and collapse all files in merge requests
With the addition of https://gitlab.com/gitlab-org/gitlab/-/merge_requests/165265, we introduced two new keyboard shortcuts to expand and collapse all files.
- Expand all files `;`
- Collapse all files `shift + ;`
These two shortcuts needs to be documented in:
1. [Merge request shortcut documentation](https://docs.gitlab.com/user/shortcuts/#merge-requests)
2. The keyboard shortcuts help modal that is available in app
issue