Restructure downloads page

[wilzbach]

This is my first attempt (WIP again) to make a better installation page, because the current one has a pretty high dropout rate (see #234).

I am not happy with this restructure yet, because:

• hard-coded direct download links (maybe @CyberShadow or @MartinNowak can make a symbolic link for "stable")?
• no icons yet (@stonemaster maybe we can special blocks/macros that take only the to be translated headers as input, e.g. like $(download_page, "Title", Description"))
• Martin's install script can also be used for Mac OS too
• Link for Fedora and openSuse doesn't help the user much (I probably should add both i386 and x86_64 links)
• no autodetection of the OS yet (show download instructions based on OS #246)

(that being said I still think it's an improvement to the status quo)

[stonemaster]

👍 Lets merge this and have the icons/macros discussion in #55