[FEATURE] Improve documentation of supported model types

[JohannesDaniel]

As issue #51 shows, users of LTR plugin struggle to find out the models to be supported by the plugin.

The plugin uses RankLib under the hood, which is not sufficiently documented itself. The LTR documentation should point out

• Which models are supported
• How do the models look like
• How can they be trained
• How can other model types be transformed

[sandeshkr419]

[Search Triage] - thanks for filing this! @JohannesDaniel Do you want to continue documenting this up?