docs: avoid copying $ prefixes from console code blocks

Rather than using `bash` for the mix of shell and output,
use `console`. That will cause the text to be marked up so
that the `$` prompt stuff won't get selected by dragging.

Enable copy button to copy the whole block, and add some
js to filter out the the $ and # prefixes from that content.

docs/javascript/fix-codeblock-console-copy-button.js

@@ -0,0 +1,42 @@
+document.addEventListener("DOMContentLoaded", function() {
+  fixCopyOnlyUserSelectable();
+});
+
+function fixCopyOnlyUserSelectable() {
+  buttonsToFix = document.querySelectorAll(
+    '.language-console button.md-clipboard');
+  if (buttonsToFix.length)
+    console.log('Fixing copy-to-clipboard text of console code-blocks.');
+  buttonsToFix.forEach((btn) => {
+    var content = extractUserSelectable(btn.dataset.clipboardTarget);
+    btn.dataset.clipboardText = content;
+  });
+}
+
+function extractUserSelectable(selector) {
+  var result = '';
+  var element = document.querySelector(selector);
+
+  // Attempt to remove the non-selectable sections based on style,
+  // but we haven't seen this work reliably...
+  element.childNodes.forEach((child) => {
+    if (child instanceof Element) {
+      var s=window.getComputedStyle(child);
+      if (s.getPropertyValue('user-select') == 'none' ||
+        s.getPropertyValue('-webkit-user-select') == 'none' ||
+        s.getPropertyValue('-ms-user-select') == 'none')
+      {
+        return;
+      }
+    }
+    result += child.textContent;
+  });
+
+  // ... so we fall back to simple but effective:
+  // remove "$ " and "# " prompt at start of lines in code
+  result = result.replace(/^[\s]?[\$#]\s+/gm, "")
+
+  // remove empty lines
+  result = result.replace(/^\s*\n/gm, '')
+  return result;
+}