Docs fixes for badges in buttons

This corrects some changes from #23060:

• Buttons must have a base and modifier class. I've added .btn-primary to fix that.
• Given the primary buttons, I've changed the badges to .badge-light for contrast.
• .bd-example and {% highlight %} don't need to be separate, so I've combined them with our {% example %} blocks designed for these situations.
• I've tweaked some docs content: added a serial comma, broke apart a rather large paragraph, and consolidated some text.

/cc @patrickhlauke