Documentation: Set page width to 900px centered

There is currently no width restriction on documentation pages,
which means long paragraphs take up the entire width of the browser
window.

On widescreen monitors this makes it uncomfortable to view the
documentation.

Fix the width to 900px, and make the content centered.

Change-Id: Iffdd4a113997685da5669e1eb2d6d5666b846357
1 file changed