Add a little bit more of margin above and below the section divider

[MarcSkovMadsen]

I love the small digestible sections in the new docs. https://pyviz-dev.github.io/hvplot/

It would more clearly separate the sections and help the reader if there was a bit more margin on top and below.

Maybe the divider needs to be a bit thicker, darker or something to help the reader clearly see the division into sections.

At the top the divider is missing and there is a scrollbar on my screen.