Proposed mutex extension.
Uncomment UNIX Mutex.
Mutex extension docblocks and several minor fixes.
composer.json file and proper class files location.
Proper files location.
Mutex cache backend WIP.
Autorelease callback improved, composer.json typo.
Autorelease callback.
Fixes:

1. Proper auto release shutdown callback.
2. Proper Mutex::releaseLock() method.
Mutex WIP.
added dabase basics docs
removed dao section from docs
Update model.md
activerecord draft
Update database-basics.md
better sentence for label example.
fixed code blocks in migration docs
Removed part about extending attributes which is not basic usage

Discussion is here:
https://github.com/yiisoft/yii2/commit/9bc5868dd1f1d843c133d73726fd3f01197a3d5d#commitcomment-3552713
Using template engines docs
more structure for doc index