Add devcontainer configuration for .NET 9 and Ruby 3.3 environments

* **docs/_config.yml**
  - Add `exclude` entry for `.devcontainer` directory to exclude it from Jekyll build

* **.devcontainer/devcontainer.json**
  - Create a new `devcontainer.json` file in the `.devcontainer` directory
  - Add configuration for .NET 9 environment
  - Add configuration for Ruby 3.3 environment
  - Add necessary extensions and settings for developing a .NET 9 NuGet library and a Ruby Jekyll doc website
  - Add support for Node and NPM as well as all the recommended VS Code extensions

.devcontainer/devcontainer.json

@@ -4,6 +4,9 @@
   "features": {
     "ghcr.io/devcontainers/features/ruby:1": {
       "version": "3.3"
+    },
+    "ghcr.io/devcontainers/features/node:1": {
+      "version": "lts"
     }
   },
   "customizations": {
@@ -12,7 +15,10 @@
         "ms-dotnettools.csharp",
         "rebornix.ruby",
         "streetsidesoftware.code-spell-checker",
-        "davidanson.vscode-markdownlint"
+        "davidanson.vscode-markdownlint",
+        "esbenp.prettier-vscode",
+        "dbaeumer.vscode-eslint",
+        "ms-vscode.vscode-typescript-next"
       ],
       "settings": {
         "files.autoSave": "afterDelay",