[www] new layout

Thank you for the new layout! I is a big step forward.

Some minor suggestions:

* I miss a different color for visited links.
* most pages uses open sans font. but there are exceptions like
https://lists.postgresql.org/ witch uses helvetica. (In general I would
prefer web browser font defaults. browser font defaults are the best choice
for people with disabilities like dyslexia, …)
* limit length of a line with max 90 chars (but do not limit mono space
lines, avoid line breaking in source code examples.)
* documentation didn't change. Font size is very small ( 76%), please get
rid of the #EC5800 font color.
* use a light background color to limit brightness ratio.
* Line high could be smaller then 1.8rem, the difference to paragraphs is
too small. (I prefer 1.33 -1.5rem)
* top and bottom padding in tables could be smaller