Fix styling issues for mermaid diagrams

What does this MR do and why?

Fixes styling issues with the rendered mermaid diagrams. This change is related to sandboxed_mermaid feature shipped in !74414 (merged).

• Keeps iframe width as full-width
• Adds a buffer height to render the height correctly

The feature flag is currently enabled for staging, and the issue can be reproduced at https://staging.gitlab.com/djadmin/test-mermaid-charts/-/issues/2.

Screenshots or screen recordings

before	after

Full page screenshot

How to set up and validate locally

1. Enable the feature flag

   Feature.enable(:sandboxed_mermaid)

2. Create an issue / MR and generate mermaid diagrams

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.