[ci skip] Update Generator.php

To comply with the docs RE namespace usage:
https://github.com/yiisoft/yii2/blob/master/docs/guide/extend-creating-extensions.md

[ci skip] Update concept-service-locator.md

[ci skip] Fixes example code symbol errors

fixes #3342

Advanced app `init` usage example typo fix

Update broken links in db-active-record.md

- update link to query builder guide
- update link to model guide
- update link to behaviors guide
- update link to database basic guide

Fixed typo in tutorial-template-engines.md

BaseMailer: strip <style> content from TextBody fix