Document escaping conflicting Mermaid syntax

[nightcityblade]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• DevOps
• Improve developer experience
• Others, please explain:

Fixes #2550

Overview of changes:

• Added a focused tip to the Mermaid plugin documentation.
• Included a copyable Gitgraph example that emits an initialization directive through a Nunjucks string expression.
• Linked to the broader raw/endraw escaping guidance.

This documents how authors can use Mermaid directives containing conflicting braces without build or rendering errors.

Anything you'd like to highlight/discuss:

The example uses MarkBind's supported Nunjucks string-expression escape syntax so the Mermaid initialization directive reaches Mermaid unchanged.

Testing instructions:

No special setup is required. Build the user guide with npm run build:ug and inspect the Mermaid plugin page.

Validation already run:

• npm run build:backend
• npm run build:ug (41 pages built successfully; existing unrelated intra-link warnings remain)
• git diff --check

Proposed commit message: (wrap lines at 72 characters)

docs: explain escaping Mermaid syntax

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Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No unrelated changes

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Reviewer checklist:

Indicate the SEMVER impact of the PR:

• Major (when you make incompatible API changes)
• Minor (when you add functionality in a backward compatible manner)
• Patch (when you make backward compatible bug fixes)

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Breaking change release note preparation (if applicable):

• To be included in the release note for any feature that is made obsolete/breaking

Give a brief explanation note about:

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• any replacement feature (if any), and
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[gerteck]

Hi @nightcityblade , thanks for your PR! We have a comprehensive PR template that we do require contributors to use.

[nightcityblade]

Thanks for the reminder. I’ve updated the PR description using the repository’s complete template and filled each applicable section and checklist item.