docs: removed problematic deps

The documentation build started failing in CI, e.g. https://gitlab.nic.cz/knot/knot-resolver/-/jobs/1626177.

I'm not sure what exactly the errors are or what has changed, but after further investigation, I found that the problems are caused by two dependencies (sphinx-tabs and json-schema-for-humans) that I don't think we need to use. So, I decided to remove them.

In my opinion, these two dependencies are not used and supported that much, so removing them will probably make our work easier in the future.