Add Mermaid-related terms to Vale vocabulary

[mintlify-development]

Summary

• Adds technical terms from PR Add ELK layout support to Mermaid documentation #3776 (mermaid-diagrams.mdx) to Vale vocabulary
• New terms: ELK, flowcharts?, subgraphs?, viewport

These are valid technical terms that would trigger Vale.Spelling false positives.

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Note

Low Risk
Only adjusts Vale vocabulary configuration; no runtime code or behavior changes beyond documentation linting output.

Overview
Updates the Vale spelling allowlist (.vale/styles/config/vocabularies/Mintlify/accept.txt) to include Mermaid-related terms (ELK, flowcharts?, subgraphs?, viewport) so documentation linting stops flagging them as false-positive misspellings.

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