Docs/CSS fix: Aallow viewport expand to user's preference

This commit removes the "max-width" property of DOM "containers";
Thus letting the viewport to expand to fill the available space.

This commit aims to bring pleasure to Docker docs' readers' eyes,
And to make them happy by letting them profit more from their large
monitors. (or use more efficiently their smaller ones).

Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)
Merge pull request #5438 from ostezer/docs-expand-viewport

Docs/CSS fix: Allow viewport expand to user's preference