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:


Start with the README's Mapping Configuration section and the settings page's Mappings tooltip/help text. Link both locations to the referenced InterSystems documentation so users can discover additional mappings such as LUT. The work is done when both locations explain that extra mappings are available and provide the documentation link.

索引モデルが issue の本文から書いたものです。