refs #3218 : add documentation

git-svn-id: svn://develop.studip.de/studip/trunk@25520 c5b85792-5220-0410-ad23-ed5b6b8ad999
refs #3239, remove plugin-migrations, use only DBManager to install/change plugin/db

git-svn-id: svn://develop.studip.de/studip/trunk@25521 c5b85792-5220-0410-ad23-ed5b6b8ad999
refs #3239, remove commented code from migration

git-svn-id: svn://develop.studip.de/studip/trunk@25522 c5b85792-5220-0410-ad23-ed5b6b8ad999