markdown: mermaid diagram are truncated when too wide

Summary

mermaid diagram are truncated when too wide

Steps to reproduce

1. Write a wide mermaid diagram, example:

   gitGraph LR:
       commit
       commit
       [...]
       commit
       commit

Example Project

gitGraph LR:
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit
    commit

What is the current bug behavior?

The mermaid diagram is truncated, so part of its information is lost

What is the expected correct behavior?

The diagram width should adapt, or it should be scrollable.

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com

Results of GitLab environment info

N/A

Results of GitLab application Check

N/A

Possible fixes

N/A

Patch release information for backports

N/A

High-severity bug remediation

N/A

References

This is maybe related to #399557 (closed) fix (!116882 (merged))