Switch sphinx theme

Preview of homepage

Main changes

• switched from sphinx-rtd to pydata-sphinx theme
• restructured documentation to take advantage of theme (compare, e.g., icet, hiphive, wulffpack, rhodent)
• selectively extended and fine-tuned documentation, in particular added cross links to API reference and tutorials in theory section, cleaned up bibliography, added images to landing page
• updated Docker image and file
• updated license information in pyproject.toml to avoid duplicating information (now information is taken from LICENSE file)
• removed unused icons
• cleaned up sphinx configuration file (doc/conf.py)
• cleaned up custom.css file
• removed outdated (and unused) letsencrypt setup file

Todo

• check that/how StaticSample.meta_data and DynamicSample.meta_data are rendered in the documentation; see !149 (merged)

Closes #201 (closed) #193 (closed)