feat(ui): add separator particle

[rtkac]

What does this PR do?

• Adds a new Separator particle to the UI registry.
• Adds a Separator example to the component docs form to demonstrate how the particle can be used.

Why?

Provides a reusable separator particle and documents its usage directly in the Separator component documentation.

Checklist

• Added the separator particle
• Added the Separator example to the docs form
• Updated generated registry files
• Ran formatting and registry sync commands

Docs

Particle

[vercel]

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