[CHA-2585] add migration notes for getstream-php (#165)

* feat: add migration guide for client setup and authentication

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* feat: create migration guide for user management operations

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* feat: create migration guide for channels (CHA-2589)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* feat: add migration guide for messages and reactions

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* feat: add migration guide for moderation (CHA-2591)

Covers: adding/removing moderators, banning/unbanning (app-level and
channel-level), shadow banning, querying banned users, muting/unmuting
users. Each section shows Before/After code blocks with key differences.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* feat: add migration guide for devices (CHA-2592)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* feat: add migration guide index with overview and quickstart (CHA-2593)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* feat: add deprecation notice to stream-chat-php README

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* chore: update link to the migration guide

* chore: update link to the migration guide

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: mogita

README.md

@@ -1,3 +1,9 @@
+> **Note:** [`getstream-php`](https://github.com/GetStream/getstream-php) is the new full-product PHP SDK for Stream, covering Chat, Video, Moderation, and Feeds with long-term support.
+>
+> **`stream-chat-php` is now in maintenance mode.** It will continue to receive critical fixes and requested features, so it is safe to keep using. However, we encourage existing users to migrate when convenient and strongly recommend that new projects start with `getstream-php`.
+>
+> Check out the [Migration Guide](https://github.com/GetStream/getstream-php/blob/39943f2219006418a41118f97b1d28f9233f72f3/docs/migration-from-stream-chat-php) for side-by-side examples covering common use cases.
+
 # Official PHP SDK for [Stream Chat](https://getstream.io/chat/)
 
 [![build](https://github.com/GetStream/stream-chat-php/workflows/build/badge.svg)](https://github.com/GetStream/stream-chat-php/actions) [![Latest Stable Version](https://poser.pugx.org/get-stream/stream-chat/v/stable)](https://packagist.org/packages/get-stream/stream-chat)