(branch_9x) start Learned Sparse Retrieval (LSR) tutorial page

[cpoerschke]

https://issues.apache.org/jira/browse/SOLR-17626 added the RawTFSimilarityFactory class targeting the (not yet released) Solr 9.9 version.

Lots of still things still TODO here, input and collaboration very welcome, e.g.

• Making a start with something.
• In a paragraph or two, what is LSR and links to a paper or two.
• Content re: training and evaluation.
• Content re: indexing.
• Content re: searching.
• Content w.r.t. the different options within Solr i.e. raw term frequency is one, payload is another, what else and how to trade-off and choose?
• Tutorial test-driving
• Relocation from branch_9x to main branch.

convenience link: https://github.com/cpoerschke/solr/blob/9x-lsr-with-raw-tf/solr/solr-ref-guide/modules/getting-started/pages/tutorial-lsr.adoc