Add new toasts examples

This should cover us nicely for how to build and customize toasts. This also basically includes how to do a dark mode (use .bg-dark) version.

• Adds a new custom content example that removes the toast header and adds a custom close icon
• Adds an example for adding content, like more buttons
• Adds docs for how to create your own color schemes

Fixes #28201 (closed) and fixes #28202 (closed).

Preview:

• https://deploy-preview-30572--twbs-bootstrap.netlify.app/docs/5.0/components/toasts/#custom-content
• https://deploy-preview-30572--twbs-bootstrap.netlify.app/docs/5.0/components/toasts/#color-schemes