Improve spell checking for documentation

The current spell checking based on codespell is very limited, as I've recently learned. For comments or variable names in the code that's probably still okay enough, but we could consider adding better spell checking for the documentation in addition to / instead of codespell.

I had a look around, at first glance this project based on hunspell looks promising: node-markdown-spellchecker in addition to codespell.

P.S.: Someone else on github already made a docker image using this - docker-markdown-spellcheck - which could be convenient for a quick integration in CI?