doc: make prompts unselectable on the manuals

doc/style.css

@@ -414,3 +414,12 @@ div.appendix .informaltable th,
 div.appendix .informaltable td {
     padding: 0.5rem;
 }
+
+/*
+  This relies on highlight.js applying certain classes on the prompts.
+  For more details, see https://highlightjs.readthedocs.io/en/latest/css-classes-reference.html#stylable-scopes
+*/
+.hljs-meta.prompt_ {
+    user-select: none;
+    -webkit-user-select: none;
+}