Docs: Write README for commands section of serverside documentation

documents/docs/networking/commands/README.md

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+# Commands
+Commands are the primary extension point of the server. 
+Every client-facing operation e.g. checking a username, sending a chat message, joining a lobby is implemented as a command. 
+Each command consists of four classes: a **Parser**, a **Request**, a **Handler**, and a **Response**.
+
+The infrastructure for routing and dispatching commands lives in the `network/` layer and is intentionally kept generic.
+The concrete implementations for each command live in `app/commands/` and are wired together at startup in `ServerApp`.
+
+![Overview of all components directly related to executing requests](/documents/images/docs/networking/commands/overview.svg)
+
+## Contents
+### Guides
+*Link to guides*
+
+### Reference
+*Link to technical deep dives*
+
+## Key Concepts
+**Each command is self-contained.**
+A command's Parser, Request, Handler, and Response all live in the same package under `app/commands/<name>/`.
+This keeps related code co-located and makes it easy to reason about a single command without navigating across multiple directories.
+
+**The `network/` layer knows nothing about specific commands.** 
+`CommandParser` and `CommandHandler` are generic interfaces. The `CommandParserDispatcher` and `CommandRouter` operate on those interfaces.
+Adding a new command **never** requires modifying infrastructure code.
+
+**Registration happens at the composition root.** 
+All commands are wired in `ServerApp` by calling `parserDispatcher.register(...)` and `commandRouter.register(...)`. 
+This keeps the wiring explicit and compiler-checked.

documents/images/docs/networking/commands/overview.puml

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+@startuml
+skinparam backgroundColor transparent
+
+package "network/ (infrastructure)" {
+    class PrimitiveRequest <<record>>
+    interface CommandParser<T extends Request>
+    interface CommandHandler<T extends Request>
+    class CommandParserDispatcher
+    class CommandRouter
+    abstract class Request
+    abstract class Response
+
+    PrimitiveRequest --> CommandParserDispatcher : routes
+    PrimitiveRequest ..> CommandParser : parsed by
+    CommandParserDispatcher --> CommandParser : routes to
+    CommandParser --> Request : parses to
+    Request --> CommandRouter : routes
+    CommandRouter --> CommandHandler : routes to
+    Request ..> CommandHandler : executed by
+    CommandHandler --> Response : creates
+}
+
+package "app/commands/<name>/ (per command)" {
+    class ExampleParser implements CommandParser
+    class ExampleRequest extends Request
+    class ExampleHandler implements CommandHandler
+    class ExampleResponse extends Response
+
+    ExampleParser --> ExampleRequest : parses to
+    ExampleRequest --> ExampleHandler : executed by
+    ExampleHandler --> ExampleResponse : produces
+}
+@enduml