Adjust docs regarding 9.0 `Neos.Neos:Plugin`

[mhsdesign]

The plugin view feature was removed with #4330

and thus we have to update the docs at:
https://docs.neos.io/guide/manual/extending-neos-with-php-flow/creating-a-plugin

currently we are experimenting over here neos/Neos.Demo#190 how to get a simple plugin working for 9.0 and once were done i think we can document it.