Clarify the difference between Core and Signal ML in the docs

[Permea-lab-admin]

Goal

Help newcomers understand what belongs in Permea Core versus Permea Signal ML.

Suggested scope

• Add a short note to README.md or an appropriate docs page.
• Explain that Core holds shared specifications, governance, and benchmark contracts.
• Explain that Signal ML holds the initial runnable ML workflow and evidence package.

Acceptance criteria

• The distinction is clear in 1-2 short paragraphs.
• The note links to the Signal ML repository if appropriate.
• The change does not move scope between repositories.

Contributor note

This is a beginner-friendly documentation issue.