added images for merge request instruction

    To https://csst-tb.bao.ac.cn/code/csst-l1/csst_proto.git

       d9f64d3..bbed3fa  main -> main

.. note::

As a developer, you can also create a merge requrest to ask code reviewers to merge your changes on GitLab.

    Click ``Merge requests`` tag, click ``

First, click ``Merge requests`` tag, click ``

    .. image:: vcs/merge_request.png

Second, select source branch and target branch, create a new merge request.

    .. image:: vcs/create_merge_request.png

The code reviewers should then view the chagnes, and decide whether the changes should be approved.

    .. image:: vcs/create_merge_request.png

If the changes are accepted, approve and merge the changes into `main` branch.

    .. image:: vcs/merge_approve.png

After the changes have been merged, code reviewers should leave comment on the changes.

    .. image:: vcs/merged.png

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