Make Merge Request Workflow into a mermaid figure

TODO

Summary

The current Merge Request Workflow figure on the webpage (https://unicado.pages.rwth-aachen.de/unicado.gitlab.io/get-involved/merge-request/) can be improved. I would suggest to use mermaid with something like this:

flowchart TD
    A[Clone 'Unicado Package' repository from GitLab] --> B[Update the submodules]
    B --> C[Checkout submodule & create new branch]
    C --> D[Commit and push to GitLab]
    D --> E[Create merge request]
    E --> F{Approved by reviewer & CI checks pass?}
    F -- No --> G[Adaption needed by developer]
    G --> F
    F -- Yes --> H[Merge to main by maintainer]

Acceptance Criteria

• figure created in mermaid style