feat(docs): migrate from .md to .rst (#4129)

docs/_static/js/include_html.js

@@ -0,0 +1,29 @@
+
+/*https://www.w3schools.com/howto/howto_html_include.asp*/
+function includeHTML() {
+  var z, i, elmnt, file, xhttp;
+  /*loop through a collection of all HTML elements:*/
+  z = document.getElementsByTagName("*");
+  for (i = 0; i < z.length; i++) {
+    elmnt = z[i];
+    /*search for elements with a certain attribute:*/
+    file = elmnt.getAttribute("include-html");
+    if (file) {
+      /*make an HTTP request using the attribute value as the file name:*/
+      xhttp = new XMLHttpRequest();
+      xhttp.onreadystatechange = function() {
+        if (this.readyState == 4) {
+          if (this.status == 200) {elmnt.innerHTML = this.responseText;}
+          if (this.status == 404) {elmnt.innerHTML = "Page not found.";}
+          /*remove the attribute, and call this function once more:*/
+          elmnt.removeAttribute("w3-include-html");
+          includeHTML();
+        }
+      }
+      xhttp.open("GET", file, true);
+      xhttp.send();
+      /*exit the function:*/
+      return;
+    }
+  }
+};