Add vscode settings for yzhang.markdown-all-in-one extension (#2813)

.gitignore

@@ -9,6 +9,7 @@ vercel_token
 # IDE
 .vscode/*
 !.vscode/extensions.json
+!.vscode/settings.json
 *.code-workspace
 
 .vercel

.vscode/settings.json

@@ -0,0 +1,4 @@
+{
+    "markdown.extension.toc.levels": "1..3",
+    "editor.formatOnSave": true
+}