Improve and simplify the layout for datatypes, with css

[panglesd]

This is the continuation of #673 by @Drup.

I added css to his modification of the layout. I also dropped the commit making the tag a div or a span depending on the number of paragraph: There might be single long paragraph, or two very small paragraph.
Instead, this is treated using css only (with flex-wrap).

You can find here, as an example, the output on a Parsetree doc where comments have been (partly) odocified, as this module contains lots of big variant and product with documentation.

Hopefully, this implements #614.

[Julow]

I like the new layout !

[jonludlam]

Thanks!