Add comprehensive documentation for icon weight and variant usage

[Copilot]

This PR addresses the missing documentation for icon weight and variant (family) settings in the Deutsche Bahn Design System's Icons.md documentation.

Problem

The original Icons.md documentation was missing crucial information about how to control:

• Icon weight/size using data-icon-weight attributes
• Icon variant/family using data-icon-variant attributes

Solution

Enhanced the packages/foundations/docs/Icons.md file with comprehensive documentation including:

Icon Weight Control

• data-icon-weight: Controls icon size with values 16, 20, 24, 32, 48, 64
• data-icon-weight-before/data-icon-weight-after: Position-specific weight control

Icon Variant/Family Control

• data-icon-variant: Controls icon style with values default, filled
• data-icon-variant-before/data-icon-variant-after: Position-specific variant control

Combined Usage Examples

<!-- 32px filled icon before text -->
<span data-icon="user" data-icon-weight="32" data-icon-variant="filled">User Profile</span>

<!-- Different styling for before and after icons -->
<span 
  data-icon="star" 
  data-icon-weight-before="20" 
  data-icon-variant-before="filled"
  data-icon-trailing="arrow_right" 
  data-icon-weight-after="16" 
  data-icon-variant-after="default">
  Favorite Item
</span>

Validation

• All existing builds and tests continue to pass
• Created and tested examples to ensure documentation accuracy
• Validated that all data attribute combinations work as documented


This enhancement provides developers with clear, actionable examples for controlling icon appearance, addressing the gap identified in the original issue.

Fixes #3961.

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Latest commit: 4b86d61

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