Update the tutorial on custom TypeMappers in the manual

[julgus]

Prior to moving all Speedment tutorials to the manual from the Wiki page, there was a tutorial on how to plug-in and use custom TypeMappers. Although, the example described in that tutorial covers usage of basic Enums which is now best handled by the dedicated Enum-plugin. Custom TypeMappers are still needed in some case though, hence this tutorial should be updated to cover a more relevant use case and be added to the manual.

Here is a reference to the current tutorial:
https://github.com/speedment/speedment/wiki/Tutorial:-Plug-in-a-Custom-TypeMapper