Doc about setting text width in custom CSS seems outdated

[KleinWurm]

The custom CSS doc adress some common modification that users might want to do. One of them is setting the text width in the main editor or in the distraction free mode.

This discussion on the main Zettlr repo is about the same topic: Zettlr/Zettlr#4609 (comment)

This issue still holds in the following version of Zettlr

• Zettlr Version: 3.5.0
• System: win32 10.0.26100 (architecture: x64)

And the proposed solution by ProodjePindakaas seems to work.

I suggest to edit the documentation Set a maximum width for the text to reflect this change.