(MAINT) Add extension suggestions

This change adds extension suggestions to the repository to ease
contributions by providing a list of helpful extensions for writing and
maintaining the documentation.

Author: michaeltlombardi

.vscode/extensions.json

@@ -0,0 +1,14 @@
+{
+  // See https://go.microsoft.com/fwlink/?LinkId=827846
+  // for the documentation about the extensions.json format
+  "recommendations": [
+    "Acrolinx.vscode-sidebar",
+    "docsmsft.docs-authoring-pack",
+    "marvhen.reflow-markdown",
+    "ms-vscode.powershell",
+    "shuworks.vscode-table-formatter",
+    "streetsidesoftware.code-spell-checker",
+    "wmaurer.change-case",
+    "yzane.markdown-pdf"
+  ]
+}