Docs guide ja start revised [ci skip]

Fixes #5480: Added defensive code to `yii\web\User::getIdentity()` to avoid potential infinite recursion

[skip ci] Update input-file-upload.md

Minor syntax changes.

[skip ci] Update input-validation.md

Minor syntax changes.

[skip ci] Update input-forms.md

Minor syntax changes.

fixes #6263 and has no problem with PHP Bug:

> it is a PHP bug: https://bugs.php.net/bug.php?id=45543
> Fixed in this commit: php/php-src@22dba2f#diff-7b738accc3d60f74c259da18588ddc5dL2996
>
> Fixed in PHP >5.4.26 and >5.5.10. http://3v4l.org/mlZX7

Update runtime-routing.md [skip ci]

Shouldn't showScriptName be false to hide entry script name?

Fixes #6103: Improved codeception config in app templates to ease testing of advanced app on multiple hosts

Fixes #4277: `yii\grid\GridView` is no longer throwing an exception in non-debug mode when results are empty and `columns` aren't defined

Fixes #5627: Added `yii cache/flush-schema` console command to flush DB schema cache of a given database connection