Improve accessibility

[jlaehne]

• Add Metadata to files
• Add automatic Sitemap
• Add language setting

Also add cite section to main page to advertise citations more prominently than just in the documentation.

What we already have implemented concerning accessibility:

• alternative text for all images
• well adapted dark and light versions
• using the optimized pydata color schemes

[jlaehne]

Fixed the build, remaining issue from #54

[ericpre]

This looks good, is it possible to reduce the width of the cite section?

[jlaehne]

This looks good, is it possible to reduce the width of the cite section?

Replaced the badge by a button set next to the text :-)

I also replaced the gitter badge as I find a button more consistent in terms of the layour