[Electric 2.0] Rewrite Shapes Guide

## Goal
Remove single-table limitations, document subqueries, ordering, limits

## Scope
- Position shapes as "real-time subscriptions to Postgres subsets"
- Document subquery support with cross-table filtering examples
- Show ordering, limits, pagination patterns
- Remove all "limitations" language about single-table
- Auth patterns using subqueries (multi-tenant auth use case)

## Current File
`/website/docs/guides/shapes.md` (276 lines)

## Dependencies
Subquery feature must be shipped/merged

## Acceptance Criteria
- User can understand shapes as real-time Postgres subscriptions
- Subquery examples are clear and practical
- Multi-tenant authorization pattern is documented
- Ordering, limits, and pagination patterns are explained

## Context
Part of Electric 2.0 launch. This is a critical doc update that shows the full power of query-driven sync.