[web/session] make sure not to modify an inner value of session.data (closes #11217958) When we modify a mutable value stored in the session's data (dict-like object) we prevent an alternative session data handler (eg. pyramid-redis-session) from detecting this update and synchronizing it in its backend (like redis).

[book] improve a bit the documentation of the database setup also remove the MySQL section.

[doc] add documentation for specializing translation of other cubes in an application cube