updates docs

[johannesjh]

• [x] update sphinx and themes. note, I also updated the minimum supported python version to 3.8 (since 3.7 is end-of-life), which allowed to install a newer version of sphinx.
• [x] following up from #74, improve the documentation on how to update dependencies
• [x] optionally: following up from #70, make it even more clear in the docs that requirements must be frozen
• [x] optionally: following up from #65, we could (shortly) describe the usage of devcontainers