glab-mr*: add pages (#7635)

pages/common/glab-mr-merge.md

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+# glab mr merge
+
+> Merge GitLab merge requests.
+> More information: <https://glab.readthedocs.io/en/latest/mr/merge.html>.
+
+- Merge the merge request associated with the current branch interactively:
+
+`glab mr merge`
+
+- Merge the specified merge request, interactively:
+
+`glab mr merge {{mr_number}}`
+
+- Merge the merge request, removing the branch on both the local and the remote:
+
+`glab mr merge --remove-source-branch`
+
+- Squash the current merge request into one commit with the message body and merge:
+
+`glab mr merge --squash --message="{{commit_message_body}}"`
+
+- Display help:
+
+`glab mr merge --help`