Doc: update Desktop screenshots with newer version

[colorchestra]

I have created some new screenshots of Joplin on Desktop for the images on the website / in the documentation. The old images show an older version of Joplin - the current versions looks so much better, we should make that immediately visible to everyone visiting the website ;)

The notes in the original screenshot were faithfully recreated, with the exception of a part that I translated from French to English. DemoDesktop.png has less text because the referenced button no longer exists, and since there is less text, I figured we can do without the arrows as it's already clear enough. The new screenshots were taken on a Linux OS, I hope that's not a problem - otherwise I may be able to dig up a Windows machine to create new screenshots on.

[tessus]

Thanks, this looks great. One thing though. The files are larger - up to 180%.

+95.2 KB (170%) docs/images/AllClients.jpg
+30.1 KB (140%) docs/images/DemoDesktop.png
+12 KB (180%) docs/images/SubNotebooks.png

Any chance you could try to decrease the size without compromising the quality too much?

[colorchestra]

Oh, I didn't think of that... AllClients.jpg is now smaller than the original (and that's the most important one imo because it's on the main website and impacts page load time), the two PNGs are also smaller than in the last commit but I didn't get them to be as small as they were before. They are still pretty small though, I think it's okay.

[tessus]

Great, thank you. I agree with your assessment. The most important is the one on the main page and you did a great job there.

And yes, PNGs are harder to minimize, but as you said, it's not that tragic on sub pages.