doc: fix issues related to page scrolling

Moved the sidebar to a fixed position and moved the main column to the
page's body, which results in back/forward navigation through hash
links and search highlight working again.

Fixes: https://github.com/nodejs/node/issues/6637
Fixes: https://github.com/nodejs/node/issues/6751
Based on: https://github.com/nodejs/node/pull/5716


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Robert Lindstaedt <robert.lindstaedt@gmail.com>