Document available record mappings.

When I installed v2.10.0 of this module, the default mappings were CLS, DFI, INC, MAC, and PTD. I did not realize adding LUT was an option and required for being able to export Data Lookup Tables. Pravin kindly pointed me to [this documentation](https://docs.intersystems.com/services/csp/docbook/DocBook.UI.Page.cls?KEY=AGITLAB#AGITLAB_settings_mappings) on [this comment](https://github.com/intersystems/git-source-control/issues/693#issuecomment-2821338291). (I have come to this module though the developer community and have never used ISC's cloud services, so I would not have thought to look there for documentation.)

I think this documentation should be linked to from the following places:
- The [Mapping Configuration](https://github.com/intersystems/git-source-control?tab=readme-ov-file#mapping-configuration) section of the readme, which does not currently hint that additional extensions are possible beyond the ones included in the screenshot.
- The settings page. The "Mappings" item has a tool tip/help text, but I think you should add some text and a hyperlink to that docs.intersystems.com page along these lines:


Commencez par la section « Mapping Configuration » du README et par le texte d’aide de l’infobulle « Mappings » de la page des paramètres. Liez ces deux emplacements à la documentation InterSystems référencée afin que les utilisateurs puissent découvrir des mappings supplémentaires tels que LUT. Le travail est terminé lorsque les deux emplacements expliquent que des mappings supplémentaires sont disponibles et fournissent le lien vers la documentation.

Rédigé par le modèle d'indexation à partir du texte de l'issue.