message: [docs] use aria-live attribute to announce copy button status
  The copy button status should be announced by the screen narrator. To
  fix this accessibility issue we need to utilize the `aria-live`
  attribute to update the status of the copy button.
  [AB#1446](https://dev.azure.com/fluidframework/235294da-091d-4c29-84fc-cdfc3d90890b/_workitems/edit/1446)
diff: // metadata at the top of the file
  const copyMessage = "Copy";
  +const copyAriaLabel = "Copy this code snippet";
  +
  const addCopyButtonsToCodeBlocks = () => {
  // Ensure the browser supports copy
  if (!document.queryCommandSupported("copy")) {
  @@ -28,14 +30,18 @@ const addCopyButtonsToCodeBlocks = () => {
  const copyBtn = document.createElement("button");
  copyBtn.className = "highlight-copy-btn";
  copyBtn.textContent = copyMessage;
  - copyBtn.setAttribute("aria-label", "Copy this code snippet");
  + copyBtn.ariaLabel = copyAriaLabel;
  + copyBtn.ariaLive = "polite";
  + copyBtn.type = "button";
  // Set a message on the button then return to the initial
  // value after 1 second
  - const flashCopyMessage = (el, msg) => {
  + const flashCopyMessage = (el, msg, label) => {
  el.textContent = msg;
  + el.ariaLabel = label;
  setTimeout(() => {
  el.textContent = copyMessage;
  + el.ariaLabel = copyAriaLabel;
  }, 1000);
  };
  @@ -46,10 +52,10 @@ const addCopyButtonsToCodeBlocks = () => {
  document.execCommand("copy");
  selection.removeAllRanges();
  - flashCopyMessage(copyBtn, "Copied!");
  + flashCopyMessage(copyBtn, "Copied!", "Copied to the clipboard");
  } catch (e) {
  console && console.log(e);
  - flashCopyMessage(copyBtn, "Failed");
  + flashCopyMessage(copyBtn, "Failed", "Failed to copy");
  }
  });
  