JSON porting tutorial

User complained that it's frustrating to port queries over that deal with JSON. They said while variant is simpler, it's different enough that you have to put a lot of thought + trial and error into it (which doesn't work well with rust compilation times).

A mini tutorial on how to migrate from SparkSQL json to Feldera SQL might do the trick.

The issue names no file, test, or entry point. Start by locating the documentation area covering Feldera SQL and JSON, then compare the requested SparkSQL-to-Feldera SQL migration cases. Done means a focused mini tutorial explains the migration with enough examples to reduce trial and error.

Written by the indexing model from the issue text.