glab-mr*: add pages (#7635)

pages/common/glab-mr.md

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+# glab mr
+
+> Manage GitLab merge requests from the command-line.
+> Some subcommands such as `glab mr create` have their own usage documentation.
+> More information: <https://glab.readthedocs.io/en/latest/mr>.
+
+- Create a merge request:
+
+`glab mr create`
+
+- Check out a merge request locally:
+
+`glab mr checkout {{mr_number}}`
+
+- View the changes made in the merge request:
+
+`glab mr diff`
+
+- Approve the merge request for the current branch:
+
+`glab mr approve`
+
+- Merge the merge request associated with the current branch interactively:
+
+`glab mr merge`
+
+- Edit a merge request interactively:
+
+`glab mr update`
+
+- Edit the target branch of a merge request:
+
+`glab mr update --target-branch {{branch_name}}`