Add DBML and D2 support from Kroki

What does this MR do and why?

Gitlab already supports drawing diagrams with Kroki, either from https://kroki.io/ or from self-hosted kroki instance.

Now Kroki also supports drawing database diagrams with the DBML language (see https://www.dbml.org) through a tool called dbml-renderer (https://github.com/softwaretechnik-berlin/dbml-renderer).

This merge request only extends the list of supported diagrams to include DBML too.

Screenshots or screen recordings

No idea how to run a modified instance of Gitlab.

How to set up and validate locally

1. Ensure your Gitlab instance is configured to use kroki, either from https://kroki.io/ or from an up-to-date self-hosted kroki instance.
2. Create any markdown text into Gitlab (issue, comment, etc.)
3. Use ```dbml to provide a diagram in DBML. Example:

      ```dbml
      Table users {
        id integer
        username varchar
        role varchar
        created_at timestamp
      }
      
      Table posts {
        id integer [primary key]
        title varchar
        body text [note: 'Content of the post']
        user_id integer
        status post_status
        created_at timestamp
      }
      
      Enum post_status {
        draft
        published
        private [note: 'visible via URL only']
      }
      
      Ref: posts.user_id > users.id // many-to-one

4. In preview or visualization, the DBML should be replaced by the same image as

MR acceptance checklist

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