Remove background color from and round corners of .gifs in docs

[MarcSkovMadsen]

Amazing new dev docs! https://pyviz-dev.github.io/hvplot/

I believe they could be improved just a bit if

• The .gifs did not have blue or grayish backgrounds
• If the corners of the gifs where just rounded a bit (not nescessary if they have same color as the background)

[MarcSkovMadsen]

As the user might also view the site on a Dark background i would recommend adding some rounding to gifs and code sections.

[MarcSkovMadsen]