docs: add documentation for mix() method in p5.strands

[Anshumancanrock]

Resolves #8441

Changes:

Added documentation for the mix() method in p5.strands. This GLSL built-in function was already implemented but missing from the reference documentation.

The documentation includes:

• Method description and usage notes
• Parameter and return type documentation
• Three examples showing color blending, vertex manipulation, and gradient creation

PR Checklist

• npm run lint passes
• Inline reference is included / updated
• Unit tests are included / updated

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