added subsection to docs index
model guide
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
added note about model validation
renamed "logging" to "log".
Refactored logging.
Fixed test breaks.
Add AR atomic operations and scenarios draft.
Fix CS issues.
Add DSN for non MySQL/MariaDB RDBMSes.
Fixes #595. Add doc on changing model scenario.