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
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