[Docs] Narrow toolbar prevents creating table 10x10

[pomek]

Is this a bug report or feature request? (choose one)

🐞 Bug report

💻 Version of CKEditor

Master

📋 Steps to reproduce

1. Execute npm run docs -- --production
2. Open built Docs > Examples > Document Editor
3. Try to insert a table 10x10

✅ Expected result

The grid should be visible.

❎ Actual result

📃 Other details that might be useful

Reproducible on Safari, Chrome, Firefox (Mac OS)

[Reinmar]

We need to fix this before the release because it looks bad.

[oleq]

Related https://github.com/ckeditor/ckeditor5-ui/issues/199#issuecomment-398358473

[oleq]

Let's remove overflow: hidden from the .document-editor container as a quick fix. It slightly breaks the bottom of the scrollbar but we can live with that.