Add Mermaid and ELK to Vale vocabulary

[mintlify-development]

Summary

• Added (?i)mermaid and ELK to the Vale vocabulary accept list
• These terms appear in the Mermaid diagrams documentation added in PR Add ELK layout support to Mermaid documentation #3776

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Note

Low Risk
Only adjusts documentation linting vocabulary; no runtime, security, or data-handling behavior changes.

Overview
Updates Vale’s Mintlify accepted vocabulary list to include ELK and case-insensitive mermaid, preventing these terms from being flagged as spelling/style errors in documentation.

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