Guidelines: add <front> element to chapter 9.1.2.1 (front matter)

[musicEnfanthen]

In chapter 9.1.2.1, the front element is not mentioned directly. This should be added to create a back link from https://music-encoding.org/guidelines/dev/elements/front.html . Now it has only a backlink from the back matter chapter 9.1.2.2.

Preferably, front and back element should be included as specDesc in both chapters, like:

<specList>
    <specDesc key="front"/>
</specList>