Add CONTRIBUTING.md, docs markdown linting and fixed dev server ports (#2714)

.markdownlint-docs.json

@@ -0,0 +1,8 @@
+{
+  "extends": "./.markdownlint.json",
+  "MD013": false,
+  "MD024": {
+    "siblings_only": true
+  },
+  "MD041": false
+}