[Feature]: support addBlankDocument flag

[passioneight]

Feature description

see https://github.com/pimcore/skeleton/blob/dca74adbbb700183269783d0d72afafa79ac86fc/config/pimcore/perspectives.example.php#L133

I had to save the a perspective and manually add the flag in the yaml file. Afterwards the checkbox was available as shown in the screenshot:

[passioneight]

Maybe also check with https://github.com/pimcore/pimcore/blob/0b6a7c151764bdc847fca5705a0f764d18adb4c7/bundles/AdminBundle/Resources/public/js/pimcore/document/tree.js#L391-L404

to see if there are more unsupported settings (e.g., addBlankSnippet).

[github-actions]

Thanks a lot for reporting the issue. We did not consider the issue as "Pimcore:Priority", "Pimcore:ToDo" or "Pimcore:Backlog", so we're not going to work on that anytime soon. Please create a pull request to fix the issue if this is a bug report. We'll then review it as quickly as possible. If you're interested in contributing a feature, please contact us first here before creating a pull request. We'll then decide whether we'd accept it or not. Thanks for your understanding.