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