[Electric 2.0] Rewrite Writes Guide

## Goal
Position TanStack DB as primary solution for optimistic mutations

## Scope
- Remove documented limitations from current guide
- TanStack DB pattern as recommended approach
- Optimistic mutations, rollback, conflict resolution
- Simplify the 4-pattern progression (make TanStack DB primary)
- Collaborative tool with optimistic mutations scenario

## Current File
`/website/docs/guides/writes.md` (297 lines)

## Dependencies
TanStack DB integration finalized

## Acceptance Criteria
TanStack DB is clearly the blessed path for writes

## Context
Part of Electric 2.0 launch. The write patterns need to be simplified and TanStack DB needs to be front and center.