Bring "Get It" page more in line with other KDE websites

Before

I like this page (I honestly think it's better than a lot of other product pages and we should bring them more in line with Kate's) but there's a couple of issues:

• What is a "unices"? Also, the "Build It" page explicitly mentions Linux and all of the download links are for Linux.
• There's a mix of complete sentences "Build it from source" and incomplete ones "Kate's release (64bit) AppImage package".
• Build it from source is repeated for every platform.

After

• Removed "unices".
• I moved "Source Code" into it's own section.
• All of the bullet points are now formalized into complete sentences. You also don't typically "install" AppImages, so I changed the wording for those too. I know AppImageInstaller exists, but not everyone uses it :-)
• Removed the extra translation string for "windows-release". This is only part of the page that has it explicitly translated like this, which is weird.

I was also going to try to separate stable and nightly builds, but it turned out kinda bad so I kept them mixed in with stable for now.