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: