Adapt the lessons from the DB manager to the Browser panel

[DelazJ]

With the demotion of DB Manager, we need to update the docs accordingly.
The lesson at https://docs.qgis.org/testing/en/docs/training_manual/databases/db_manager.html should then be reviewed. And considering that the DB Manager is being replaced by the Browser panel, it would make sense to adapt the instructions to the Browser panel and update https://docs.qgis.org/testing/en/docs/training_manual/databases/db_browser.html accordingly.