Enhance restricted editing feature guide

[jodator]

📝 Provide a description of requested docs changes

The restricted editing feature guide needs polishing.

• Missing plugins in the demo. E.g. tables?
• explain that two modes should not be collaborative (collaboration session from restricted mode should not be made with a standard mode) - Restricted editing should not mix attributes with markers #5822 (comment)
• Add keyboard shortcuts to the Keyboard support guide
• make it clear that in the current version paste does not work in the restricted editing mode.