Fix documentation

1. Important fix

Equations on the current website are broken, e.g. http://ift.pages.mpcdf.de/nifty/user/volume.html but by commenting out the sphinx.ext.imgmath' extension in the conf.py this is resolved. (at least on my machine) @parras : was there are specific reason for rendering the equations to images. If not and if it generally works without this line I would stick with this fix.

2. other fixes:

found some broken links to the ducc repository and matched some style.

@mtr , @gedenhof : I marked this MR as Draft because I'm not sure if we need further adjustment in the first fix.