[DOC] Add a link to the LTR demo site to clarify the example

[sstults]

What do you want to do?

• [ x] Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request. Provide a summary of the request.
There used to be a link to the hello-ltr demo project that provides context for the example queries. The link was broken when that project was moved out of the source tree of the LTR plugin and into its own project. The broken link was found when the documentation was added to this repo, but since the code it referred had moved the link was just removed.

I would like to add this link back at the beginning of examples where references to the index schema would be helpful. In particular it would be helpful at the top of logging-features.md.

Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.
2.17.1

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
opensearch-project/opensearch-learning-to-rank-base#43