cheatsheet: improve PDF appearance and add PDF for v5.0.

[AlbertoGP]

The HTML and CSS changes keep the width inside the page without scaling, allow text wrapping, and avoid page breaks inside sections as much as possible. I moved the section titles inside the div to avoid them ending up some times in a different page from the content.

The second commit adds the PDF produced by Chromium, which fixes #69.

[AlbertoGP]

The rule #logo { content:url(../common/img/raylib_logo.png); } is so that the logo gets included automatically in the PDF. Otherwise it is omitted because it is a background image.

[raysan5]

Thanks for the review but I'm afraid I prefer to keep the old PDF style with the background colors and wider sections. Also the cheatsheet page is being reviewed on another PR.