architecture document with an overview diagram

Author: IronCore864

docs/architecture.md

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+# DevStream Architecture Summary
+
+This document summarizes the main components of DevStream and how data and requests flow between these components.
+
+## DevStream Request Flow
+
+The following diagram shows an approximation of how DevStream executes a user command:
+
+![DevStream Architecture Diagram](./images/architecture-overview.png)
+
+## CLI (The `devstream` Package)
+
+Every time a user runs the `dtm` program, the execution transfers immediately into one of the "command" implementations in the [`devstream`](https://github.com/merico-dev/stream/tree/main/cmd/devstream) package, in which folder all commands' definitions reside. Then, each command calls the corresponding package under [`internal/pkg`](https://github.com/merico-dev/stream/tree/main/internal/pkg).
+
+The flow illustrated above applies to the main DevStream commands like `dtm install`, `dtm uninstall` (_TODO_), and `dtm reinstall` (_TODO_). For these commands, the role of the command is to parse all the config, load each plugin, and call the [predefined interface](https://github.com/merico-dev/stream/blob/main/internal/pkg/plugin/plugin.go#L12).
+
+## Configuration Loader
+
+Model types in package [`config`](https://github.com/merico-dev/stream/blob/main/internal/pkg/config/config.go) represent the top-level configuration structure.
+
+## State Manager
+
+_TODO_
+
+## Plugin Engine
+
+The plugin engine has various responsibilities:
+
+- making sure the required plugins (according to the config) are present (_TODO_)
+- generate a plan according to the state (_TODO_)
+- execute the plan by loading each plugin and running the desired operation
+
+_Note: there will be a rework of the engine to generate and execute a plan according to the state manager, which hasn't been implemented yet._
+
+## Plugins
+
+A _plugin_ implements the aforementioned predefined interfaces.
+
+It executes operations like install, reinstall (_TODO_), uninstall (_TODO_), and stores state (_TODO_).