Add markdownit for automatic link embeds

Convert Alerts page to markdown as example

Bump some nuxt-related dependencies so markdownit plugin works

Docs: https://github.com/nuxt-community/modules/tree/master/packages/markdownit#usage

If we don't want separate files we could use <template lang="md"> in Vue files instead

Intersects with https://gitlab.com/gitlab-org/design.gitlab.com/issues/67