Update image and description in immcantation.org

[ssnn]

I have updated immcantation.org to add dowser in the core packages side menu and main section. I used a temporary image. If you want to change it, replace /docs/_static/dowser.png. You wan also update the description in docs/index.rst to add a little more detail.

[ssnn]

I updated the image in 7ff5b5b