Update merge_requests.md with a simpler way to check out a merge request

## What does this MR do?

This MR updates `merge_requests.md` with a simpler way to check out a merge request 

The previous way was put under the "By modifying `.git/config` for a given repository" section. The disadvantage of the old way is that one has to modify `.git/config` for each repository that one uses. The new way was put under the "By adding a git alias" section. One has to create this git alias once, and then it will work from any repository, without having to modify `.git/config` over and over again.

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

One should double check that the link provided by the "Check out branch" button in a MR, i.e. https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/workflow/merge_requests.md#checkout-merge-requests-locally, still works. I think it will, since I didn't modify the "Checkout merge requests locally" section name.

## Why was this MR needed?

This MR was needed because the old way of checking out a merge request locally requires to modify `.git/config` in all repositories that one has on a computer, which is a tedious task. The new way just requires adding a simple git alias, once. And the `git mr` command then works for all repositories.

## What are the relevant issue numbers?

I haven't created an issue for this.

## Screenshots (if relevant)

N/A

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added (N/A)
- Tests
  - [ ] Added for this feature/bug (N/A)
  - [x] 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)

See merge request !5944