[book] Update documentation for new repoapi
Quite a few things change in 3.19:
- repoapi instead of dbapi
- ClientConnection / Connection / Session rework
- web authentication process
- test APIs
Closes #3638793
usr/lib/PY_VERSION/*-packages/cubicweb/entities/
usr/lib/PY_VERSION/*-packages/cubicweb/ext/
usr/share/cubicweb/cubes/
usr/lib/PY_VERSION/*-packages/cubicweb/*.py