[ci skip] Adds start-gii-crud-preview image to the gii guide

Used a single database and a single CLI entry point for all test types. Added faker to basic app dev requirements.

structure-extensions.md translated to Russian[skip-ci]

Fix PHPDoc

fixes typos in start-databases.md

Unused InvalidConfigException

fix bugs of BlameableBehavior in console application. In console application, \yii\console\Application doesn't have `getUser()` method. Use BlameableBehavior in console application will cause an exception.

fix a typo.

Rewritten testing introduction, added environment setup to README.md, removed empty pages that were not referenced