plaintext-de: Add copy for German short version
plaintext-de: Abstract
plaintext-de: Plain text format chapter
plaintext-de: Version control chapter
plaintext-de: Remove Where to go from here chapter

This chapter is superfluous in this short version
plaintext-de: Remove appendices

These are superfluous in the short version
plaintext-de: Remove "openLilyLib Tutorials" dependencies

This version is an independent document.
Especially it's in German, and I didn't see a way to modify the main document
to support languages.
And I want to use my non-free fonts for that specific purpose.
So I keep the file in the tutorials directory but make it (partially) independent