doc(api): Add the doc to explain why the cube API is important here

5 jobs for branch/default in 1 minute and 29 seconds (queued for 1 second)
Name Stage Failure
mypy Lint
/builds/cubicweb/owl2yams/.tox/mypy/bin/python -m pip install types-setuptools

Found 5 errors in 1 file (checked 5 source files)
ERROR: InvocationError for command /builds/cubicweb/owl2yams/.tox/mypy/bin/mypy --install-types --non-interactive --ignore-missing-imports owl2yams (exited with code 1)
___________________________________ summary ____________________________________
ERROR: mypy: commands failed
Cleaning up file based variables
ERROR: Job failed: command terminated with exit code 1