Desktop: Add support for multiple instances (#11963)

readme/dev/spec/multiple_instances.md

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+# Multiple instance support
+
+Joplin Desktop supports multiple instances through **profile locking** and **IPC messaging**.
+
+### Profile Locking
+
+- A lock file is updated every `x` seconds by each instance.
+
+- If a valid lock exists, the new instance sends a message to all running instances and closes. This message prompts the other active instance to move to the front.
+
+### IPC Messaging
+
+- IPC is implemented using lightweight HTTP servers in each instance, communicating via `POST` requests.
+
+- When a message is sent, the implementation automatically discovers running IPC servers.
+
+### Instance Differentiation
+
+- The `--alt-instance-id` flag must be used to launch an alternative instance. This disables services like the Web Clipper.
+
+- The `altInstanceId` setting is consulted by the application to determine its type (main or alternative instance).
+
+### Current Limitations
+
+The system is designed to handle multiple instances but currently supports only two through the UI: a **main instance** and an **alternative instance**.