[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
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html