Spec: Describe patches, merge requests, opening a merge request (#185)
    
    - Vocabulary: Add the 'Patch' type
    - Modeling: Describe how to represent a Patch and a Merge Request
    - Behavior: Describe how to open a Merge Request
    
    See #169 (open in private browsing if you get an error) for more info about why this PR is designed the way it is.
    
    Co-authored-by: fr33domlover <fr33domlover@riseup.net>
    Reviewed-on: https://codeberg.org/ForgeFed/ForgeFed/pulls/185