doc: add devcontainer doc

Author: necusjz

doc/assets/codegen_cli_generator_1.png

@@ -0,0 +1,50 @@
+GitHub Codespace gets you up and coding faster with fully configured, secure cloud development environments native to GitHub.
+
+## Create a codespace
+Currently, the dev container is available in [azure-cli-extensions](https://github.com/Azure/azure-cli-extensions), as it's the repository that our contributors mainly contribute to. However, we can still contribute codes to [azure-cli](https://github.com/Azure/azure-cli) based on it.
+
+In the repository of GitHub, we can create a codespace as follows:
+![](/doc/assets/dev_container_entry.png)
+
+## Login account of GitHub
+Once codespace is started (take a couple of minutes), we will see the following prompt in the integrated terminal:
+![](/doc/assets/github_account_login.png)
+
+It will help us log in to our account in an interactive way, after logging in, we don't need to do it again in the fresh terminals:
+![](/doc/assets/github_already_login.png)
+
+## Run ./easy_setup
+As we can see, Python virtual environment has been automatically activated. We can directly execute the script to setup the development environment:
+```bash
+$ ./easy_setup
+```
+
+Usually the whole process will take several minutes. When the following prompt is displayed, it means the whole setup process has been completed:
+![](/doc/assets/dev_container_finished.png)
+
+## Fundamentals of codegen tool
+Once the development environment is setup, we can launch the codegen tool via:
+```bash
+$ aaz-dev run -c azure-cli -e azure-cli-extensions -s azure-rest-api-specs -a aaz
+```
+> Above 4 parameters are the paths of the repositories, please feel free to modify them if you have other needs, e.g., your swagger source is `azure-rest-api-specs-pr`.
+
+We can refer to the full document of codegen tool at https://azure.github.io/aaz-dev-tools/. However, the following will briefly introduce the 2 pages that we often interact with.
+
+### Workspace Editor
+[Workspace Editor](https://azure.github.io/aaz-dev-tools/pages/usage/workspace-editor/) is usually the start page to develop our command modules. We can open or create a workspace at:
+![](/doc/assets/codegen_workspace_editor_1.png)
+
+If everything is selected correctly, we will be redirected to the following UI. We can interactively edit our ideal command line interface based on that.
+![](/doc/assets/codegen_workspace_editor_2.png)
+
+### CLI Generator
+When we have completed all the editing in Workspace Editor and clicked EXPORT in its upper right corner. It's ready to switch to [CLI Generator](https://azure.github.io/aaz-dev-tools/pages/usage/cli-generator/) to generate code of Azure CLI.
+
+First, we need to select the target for generating code. If you don't know where to generate the code, usually Azure CLI Extension is all you need.
+![](/doc/assets/codegen_cli_generator_1.png)
+
+Next, we can find the commands that we modified before in the following UI. Check the corresponding checkboxes, then click GENERATE in the upper right corner, and the code will be generated.
+![](/doc/assets/codegen_cli_generator_2.png)
+
+> More advanced guides, such as [testing](https://azure.github.io/aaz-dev-tools/pages/usage/command-usage-testing/) and [customization](https://azure.github.io/aaz-dev-tools/pages/usage/customization/), are also explained in the full document.