Organized setup instructions (#115)

* Organized setup instructions

* update tets

* link checker action

* raise error upon broken link

* fix links

* fix links

* delete duplicated paragraph

setup/03_optional-docker-environment/.devcontainer/devcontainer.json

@@ -0,0 +1,19 @@
+{
+  "name": "LLMs From Scratch",
+  "build": {
+    "context": "..",
+    "dockerfile": "Dockerfile"
+  },
+  "runArgs": ["--runtime=nvidia", "--gpus=all"],
+  "customizations": {
+    "vscode": {
+      "extensions": [
+        "ms-python.python",
+        "ms-azuretools.vscode-docker",
+        "ms-toolsai.jupyter",
+        "yahyabatulu.vscode-markdown-alert",
+        "tomoki1207.pdf"
+      ]
+    }
+  }
+}