feat: improve six-sidebar component

[JoelLeupp]

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation, readme or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

• Add support for icons in six-sidbar-item
• fix spacing for six-sidbar sub-items
• added sidbar helper directives for angular: ActiveSidebarItemGroupDirective and ActiveSidebarItemDirective
• added a nested sidbar menu in the angular demo app to showcase the helper directives

📝 Checklist

• It's submitted to the main branch
• I have updated the documentation accordingly.
• All tests are passing
• New/updated tests are included
• I have updated the "upcoming" section inside docs/changelog.md explaining the changes I contributed