Add special markers to document-schema

We should add some special markers to the document-schema:

• awaiting / open / open request
• todo
• marked (in general)

The reason should be obvious, we can show these marked documents on the dashboard so the user can "pin" some documents that are needed within near future.

I would prefere to implementing only a marked marker for now, since this should be sufficient.

If we later think, that 1-2 other markers are needed, we can still implement them.