Fixed 404 download link on index html page css changes

I changed the width of the page to max width 600px to accommodate a PRE
style that better represents “code” commands and max width to better
accommodate mobile devices. I made many more changes to mobile stylings
for the page so it scales really well for smaller devices now. I also
proof-read the content of the page and corrected any English translation
errors for you and unified some of the messages you had on the page
adding links where appropriate.
Border between paragraphs under #api and usage is h3