docs/guide/input-file-upload.md - revised a bit [ci skip]

/docs/guide-ja/input-file-upload.md added [ci skip]

Fixes #6434: Added `yii\behaviors\SluggableBehavior::immutable` to support keeping the generated slug unchanged

Spanish guide Query Builder [skip ci]

/docs/guide-ja/input-validations.md added [ci skip]

docs/guide/input-validation.md - typo [ci skip]

[skip ci] Update start-looking-ahead.md

[Uk] Guide corrections [skip ci]

Adding the tag titled "yii2" on Stackoverflow to Community list.

fix  mistake small  [skip ci]

Fixes #6514: advanced application SignupForm::signup() should not return User model if saving failed