Add SQLite JSON querying guide

[bean1352]

Added comprehensive guide for querying JSON data in the client SQLite database

• Explains JSON storage as strings in SQLite when synced from backend (MongoDB, PostgreSQL, MySQL)
• Documents -> vs ->> operators with clear examples showing quoted vs unquoted output
• Covers json_extract() and json_each() for flattening and querying nested arrays/objects
• Includes real-world examples: filtering, aggregating etc
• Shows delimiter-separated value queries using instr() pattern
• Performance optimization tips: indexing JSON columns in AppSchema, filtering before expanding, using EXISTS
• Documents common gotchas: NULL handling, type casting
• Provides reference for useful JSON functions (json_array_length, json_valid, json_type, etc.)

[benitav]

this is great! 🔥 One suggestion I have is to link to this reference from other places in the docs where this use case would come up, e.g. two that come to mind are https://docs.powersync.com/usage/use-case-examples/custom-types-arrays-and-json (maybe even consolidate these 2 pages as there seems to be a lot of overlap), and where we mention JSON on https://docs.powersync.com/usage/sync-rules/types

[michaelbarnes]

Overall, this is a great addition. I've left a few comments around wording and a few links back to SQLite docs for easy reference.