[Docs] Add tabpane example in contributing guide

[leecalcote]

Current State

This shortcode is missing in the contributing guide's set of shortcode examples

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Contributor Guide and Resources

• 📚 Instructions for contributing to documentation
  • Layer5 documentation site and source
• 🎨 Wireframes and designs for Layer5 site in Figma (open invite)
• 🙋🏾🙋🏼 Questions: Layer5 Discussion Forum and Layer5 Community Slack