Add narrow page option

Many pages feature a lot of text, which becomes cumbersome to read on
maximized browser windows on smaller screens (< 1920x1080). To improve
readability this commit introduces front-matter parameter to reduce the
page's text width to max. 800px. Tables and images are exlucded and not
narrowed to max. 800px to avoid problems.

The feature can be enabled by adding `isNarrowPage = true` to a page's
front matter.

Signed-off-by: Sietze van Buuren <Sietze.vanBuuren@de.bosch.com>