📝(documentation) add collaboration architecture doc

Documentation to describe the collaboration
architecture in the project.

Author: AntoLC

docs/collaboration-architecture.md

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+# Architecture Overview
+
+This architecture showcases different ways for clients to interact with a **Hocus Pocus Server** (a [Y.js](https://github.com/yjs/yjs) provider) through either WebSockets, HTTP fallbacks, or Server-Sent Events (SSE) when WebSockets are not available.
+
+**Main Components**:
+
+- **Client**: The front-end application or user agent.
+- **Nginx**: A reverse proxy handling incoming requests, forwarding them to the appropriate services, and managing SSL/TLS termination if needed.
+- **Auth Sub Request (Django)**: Handles authentication/authorization, ensuring requests have valid credentials or permissions.
+- **Hocus Pocus Server**: The core collaborative editing server (powered by [Y.js](https://github.com/yjs/yjs) libraries) that manages document state and synchronization.
+- **Express**: Fallback server to handle push or pull requests when WebSocket connections fail.
+- **SSE**: A mechanism (Server-Sent Events) for real-time updates when WebSockets are unavailable.
+
+## Mermaid Diagram
+
+```mermaid
+flowchart TD
+    title1[WebSocket Success]-->Client1(Client)<--->|WebSocket Success|WS1(Websocket) --> Nginx1(Ngnix) <--> Auth1("Auth Sub Request (Django)") --->|With the good right|YServer1("Hocus Pocus Server")
+    YServer1 --> WS1
+    YServer1 <--> clients(Dispatch to clients)
+
+    title2[WebSocket Fails - Push data]-->Client2(Client)---|WebSocket fails|HTTP2(HTTP) --> Nginx2(Ngnix) <--> Auth2("Auth Sub Request (Django)")--->|With the good right|Express2(Express) --> YServer2("Hocus Pocus Server") --> clients(Dispatch to clients)
+
+    title3[WebSocket Fails - Pull data]-->Client3(Client)<--->|WebSocket fails|SSE(SSE) --> Nginx3(Ngnix) <--> Auth3("Auth Sub Request (Django)") --->|With the good right|Express3(Express) --> YServer3("Listen Hocus Pocus Server")
+    YServer3("Listen Hocus Pocus Server") --> SSE
+    YServer3("Listen Hocus Pocus Server") <--> clients(Data from clients)
+```
+
+---
+
+## Detailed Flows
+
+### 1. WebSocket Success
+1. **Client** attempts a WebSocket connection.
+2. **Nginx** proxies the WebSocket connection through the **Auth Sub Request (Django)** for authentication.
+3. Once authenticated, traffic is routed to the **Hocus Pocus Server**.
+4. The server can broadcast data to all clients connected through WebSockets.  
+   - Note: The path `YServer1 --> WS1` indicates the two-way real-time communication between the server and client(s).
+
+### 2. WebSocket Fails — Push Data (HTTP)
+If WebSocket connections fail, clients can **push** data via HTTP:
+1. **Client** detects WebSocket failure and falls back to sending data over **HTTP**.
+2. **Nginx** handles HTTP requests and authenticates them via the **Auth Sub Request (Django)**.
+3. After successful authentication, the requests go to an **Express** server.
+4. The **Express** server relays changes to the **Hocus Pocus Server**.
+5. The **Hocus Pocus Server** dispatches updated content to connected clients.  
+
+### 3. WebSocket Fails — Pull Data (SSE)
+For continuously receiving data when WebSockets fail, the client can **pull** data using SSE:
+1. **Client** sets up an **SSE** connection.
+2. **Nginx** proxies the SSE stream request through the **Auth Sub Request (Django)** for authentication.
+3. Once authenticated, the **Express** server listens to the **Hocus Pocus Server** for changes.
+4. The server then sends updates back to the **Client** through SSE in near real-time.
+
+---
+
+## Component Responsibilities
+
+| **Component**               | **Responsibility**                                                                     |
+|-----------------------------|-----------------------------------------------------------------------------------------|
+| **Client**                  | Initiates connections (WebSocket/HTTP/SSE), displays and interacts with data           |
+| **Nginx**                   | Acts as a reverse proxy, routes traffic, handles SSL, and passes auth sub requests     |
+| **Auth Sub Request (Django)** | Validates requests, ensuring correct permissions and tokens                           |
+| **WebSocket**               | Real-time two-way communication channel                                                |
+| **HTTP**                    | Fallback method for sending updates when WebSockets are not available                  |
+| **Express**                 | Fallback server for handling requests (push/pull of data)                              |
+| **SSE**                     | Mechanism for real-time one-way updates from server to client                          |
+| **Hocus Pocus Server**      | Core Y.js server for collaboration, managing document states and synchronization       |
+
+---
+
+## Why This Setup?
+
+- **Reliability:** Ensures that when a user’s browser or network environment does not support WebSockets, there are fallback mechanisms (HTTP for push updates and SSE for server-initiated updates).
+- **Scalability:** Nginx can efficiently proxy requests and scale horizontally, while the authentication step is centralized in Django.
+- **Security:** The Auth Sub Request in Django enforces proper permissions before data is relayed to the collaboration server.
+- **Real-time Collaboration:** The Hocus Pocus Server provides low-latency updates, essential for collaborative editing, supported by [Y.js](https://github.com/yjs/yjs).
+
+---
+
+### Contributing
+If you have any suggestions or improvements, feel free to open an issue or submit a pull request.
+
+**Thank you for exploring this architecture!** If you have any questions or need more detailed explanations, please let us know.