Replace all screenshots in the documentation with high DPI screenshots

[ThiloteE]

Cause: Users of smartphones have a tiny screen and require to zoom in. Screenshots are unfortunately at this level of zoom full of pixels. Additionally, text looks ugly or cannot be read at all.

Priority: low
JabRef cannot be used on a phone, thus we can deduce all users of JabRef will have a Desktop PC or a Laptop available and the share of users looking at the JabRef documentation via their phone must be very low.