Docs: group together reusable CSS for examples in a single stylesheet

Description

• Gathering reusable CSS from Bootstrap examples in site/layouts/_default/examples.html
• This stylesheet defines .b-example-divider, .b-example-vr and .bi used in several examples.

Motivation & Context

This change could help the maintenance of the examples by gathering in a single place some CSS definitions reused several times in Bootstrap examples.

Types of changes

• Refactoring (non-breaking change)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)

Related issues

Live previews

For non-regression testing:

• Dropdowns example
• Features example
• Footers example
• Headers example
• Heroes example
• List groups example
• Modals example
• Sidebars example