Examples: restore sidebar dropdown focus outline

[Kevinjohn]

Description

Restores the default focus outline for dropdown toggles in the sidebars example.

Motivation & Context

The sidebars example removed outlines from every .dropdown-toggle, including focused dropdown triggers. The headers example already limits the same outline reset to .dropdown-toggle:not(:focus), preserving the keyboard focus indicator.

Expected: focused sidebar dropdown triggers retain a visible outline.
Actual: focused sidebar dropdown triggers have outline: 0 with no replacement style.

This updates the sidebars selector to match the existing headers example pattern.

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to an example
• I have updated the example accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-42710--twbs-bootstrap.netlify.app/

Related issues

None.

Checks

• git diff --check
• npm exec -- prettier --config site/.prettierrc.json -c site/src/assets/examples/sidebars/sidebars.css
• npm run docs-build