Redesign and update index page

In my opinion the current homepage has a few issues:

• not nice-looking colors
• Outdated screenshots
• Old Plasma 5.4-ish design
• Misleading and old KDE group photo which somehow suggests all those people were working on Plasma Mobile :)

My suggestion is to make the landing page a lot simpler, since we have all the information from it on other pages already. The background image has been updated just like the screenshot on the phone (Unfortunately I don't have the original files the picture was made from, so it might not look very good after this basic editing using GIMP). Btw I also added the xcf file, so updating the screenshot later should be easy now. This changes would also remove some more wordpress-generated code, which makes further editing a lot easier.

I'm not completely sure about this changes, so feel free to suggest different options (including staying with the current design and content) :)