MucIntro: Add missing pieces

For MucIntro there are two essential pieces missing currently:

1. The tagline above the heading: https://patternlab.muenchen.space/?p=components-intro-vertical-article
2. A seperator that can be toggled to be visible between the heading and the content

![image](https://github.com/it-at-m/muc-patternlab-vue/assets/10800158/70b9ce27-b2d5-442c-bc87-fcb4de71c09c)


