docs: Document custom color picker for sticky notes

[LudwigGerdes]

Add documentation for the new custom color picker feature that allows users to select any color for sticky notes beyond the 7 preset colors.

• Add custom colors section with step-by-step instructions
• Include screenshots showing color picker popover and custom color modal
• Document recent colors feature (up to 8 saved)
• Explain theme-aware borders for custom colors

related to: https://github.com/n8n-io/n8n/pull/23699

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Summary by cubic

Documented the new custom color picker for Sticky Notes, so users can pick any color beyond the 7 presets. Added simple steps, explained recent colors (up to 8) and theme-aware borders, and included screenshots of the picker popover and the custom color modal.

^{Written for commit 82d59279690cf250a75f19a56f737947b6febedd. Summary will update on new commits.}

[CLAassistant]

All committers have signed the CLA.

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