Replace fonts with open-source fonts.

[danwos]

For the cover, css is using "Helvetica Neue" as font, which is a proprietary font and not available on all systems.

For the rest, we already provide fonts together with this package.

We need to do:

• Replace the fonts from the cover with an open-source font
• Add the new-cover font files to the fonts folder.
• Check what the license of the other, currently used fonts is and maybe replace

[danwos]

FiraSans is free: https://fonts.google.com/specimen/Fira+Sans/about

Maybe use another google font for the cover. Or FiraSans is also good looking for the cover

[danwos]

Replaced "Helvetica New" with "Work Sans". Sphinx-SimplePDF also delivers this font now. So we don't use any system / pre-installed fonts anymore.