Add use-case oriented guides

[tobiasdiez]

The collection pages "Collect", "Organize", etc are currently empty. I think this is a good place to add short guides that describe different use cases. For example, under the Collect page one should answer questions like "I have the book in front of me, how do I add it?" or "I'm found an interesting article while browsing the web, how do import it into JabRef?". The desription shouldn't be too in depth, just give a brief outline how to do each action and link the later articles for more details, for example, "manually using the information on the copyright page or based on ISBN" and "using the browser extension".

See e.g mendeley guides or this article or A guide to using JabRef

[koppor]

The (German) guide at https://github.com/JabRef/manuals.jabref.org/tree/master/de could also be a good start.