[Electric 2.0] Create Collections & Queries Guide

## Goal
Frontend data modeling patterns with TanStack DB

## Scope
- How to model data loading in frontend apps
- Multiple patterns (co-located queries, progressive loading, pagination)
- When to use each pattern
- TanStack DB integration examples
- Progressive data loading for AI apps scenario

## New File
`/website/docs/guides/collections-queries.md` (name TBD - open to suggestions)

## Dependencies
TanStack DB integration must be solid

## Acceptance Criteria
Covers all major frontend data loading patterns

## Context
Part of Electric 2.0 launch. This guide shows developers how to think about data loading in a sync-first world.

## Open Question
What should this guide be called? "Collections & Queries"? "Data Loading Patterns"? "Frontend Queries"?