Add notes about text indexes

[matthewhelmke]

This adds an EXPERIMENTAL label to the native text index with a link to the Lucene text index. Then, it renames the Lucene page to Text index with a note that this is preferred. Finally, it adds a link at the top of the Lucene page to help readers skip the background info and go straight to the procedure for enabling.

[npawar]

Thanks @matthewhelmke ! would you also please update the index here: https://docs.pinot.apache.org/basics/indexing We can simply remove Native Text index from the list there

[matthewhelmke]

@npawar , I'll take care of it in the next commit, then I'll get this merged. :)