add "replacement" to frontmatter for deprecated components

[brian-smith-tcril]

When looking at https://github.com/openedx/paragon/pull/2492 it seemed like adding a link to the recommended replacement component would be a good idea. We don't, however, parse mdx.frontmatter.notes as markdown when rendering it in the alert.

Instead of trying to support arbitrary markdown in the alert, I think it would make sense to add a "replacement" field to the frontmatter for use with deprecated components, so we can have a nicely styled link to the docs page for the component we recommend consumers use instead.