Important: #102130 - Optimizing T3D Import/Export module

See forge#102130

Description

The backend Import/Export module is based on code that is very hard to maintain, and is tightly coupled with both the DataHandler and RefIndex routines.

To improve maintainability and reliability in these areas, parts of the Import/Export modules need to be refactored as an ongoing effort.

The Documentation of the Import/Export module now addresses these challenges.

The ongoing refactoring may affect importing dumps from older versions of TYPO3. Importing these dumps may lead to missing data, also due to changes made longer ago (i.e. pages_language_overlay no longer existing, changes in workspaces, ...).

When performing an import of outdated TYPO3 versions, thoroughly check the generated warnings and errors, so that you can either recreate or upload missing data manually.