Based on https://github.com/twbs/bootstrap/pull/28265#issuecomment-754277950.
Adds a heading and example of using the flex utilities in a barebones example. Using the heading provides us an easy way to raise awareness of the migration by showing up when folks search for the v4 component in v5's docs.
/cc @XhmikosR
Preview: https://deploy-preview-33207--twbs-bootstrap.netlify.app/docs/5.0/utilities/flex/#media-object