Fixes mermaid diagrams not rendering on self-hosted docs

What does this MR do and why?

Adds a new JS route to fix a bug where self-hosted docs would not render mermaid diagrams.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Visit https://gdk.test:3000/help/user/project/merge_requests/homepage/ to see the mermaid diagram render correctly

MR acceptance checklist

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