[TASK][DOC] Split administrator chapter into subchapters

Change-Id: If0e2abdd73f6b88db31b82cfe288bd1ead5241b2
Reviewed-on: http://review.typo3.org/32167
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Tested-by: Xavier Perseguers <xavier@typo3.org>