Style warning note in book

This adds some style to the warning note in the book.  It looks similar
to the v1 book style, but with a more semantic structure.

docs/book/src/cronjob-tutorial/running-webhook.md

@@ -57,14 +57,18 @@ kubectl create -f config/samples/batch_v1_cronjob.yaml
 You can also try to create an invalid CronJob (e.g. use an ill-formatted
 schedule field). You should see a creation failure with a validation error.
 
-<aside>
-Note: If you are deploying a webhook for pods in the same cluster, be
+<aside class="note warning">
+
+<h1>The Bootstrapping Problem</h1>
+
+If you are deploying a webhook for pods in the same cluster, be
 careful about the bootstrapping problem, since the creation request of the
 webhook pod would be sent to the webhook pod itself, which hasn't come up yet.
 
 To make it work, you can either use [namespaceSelector] if your kubernetes
 version is 1.9+ or use [objectSelector] if your kubernetes version is 1.15+ to
 skip itself.
+
 </aside>
 
 [namespaceSelector]: https://github.com/kubernetes/api/blob/kubernetes-1.14.5/admissionregistration/v1beta1/types.go#L189-L233

docs/book/theme/css/general.css

@@ -397,3 +397,43 @@ main > details.collapse-code > summary::after {
     fill: black;
     stroke: none;
 }
+
+/* notes */
+aside.note {
+    border: 1px solid var(--searchbar-border-color);
+    border-radius: 3px;
+}
+
+aside.note > * {
+    margin-left: 1em;
+    margin-right: 1em;
+}
+
+/* note title */
+aside.note > h1 {
+    border-bottom: 1px solid var(--searchbar-border-color);
+    margin: 0;
+    padding: 0.5em 1em;
+    font-size: 100%;
+    font-weight: normal;
+}
+
+/* warning notes */
+aside.note.warning > h1 {
+    background: var(--warning-note-background-color, #fcf8f2);
+}
+aside.note.warning > h1::before {
+    /* TODO(directxman12): fill in these colors in theme.
+     * If you're good with colors, feel free to play around with this
+     * in dark mode. */
+    content: "!";
+    color: var(--warning-note-color, #f0ad4e);
+    margin-right: 1em;
+    font-size: 100%;
+    vertical-align: middle;
+    font-weight: bold;
+    padding-left: 0.6em;
+    padding-right: 0.6em;
+    border-radius: 50%;
+    border: 2px solid var(--warning-note-color, #f0ad4e);
+}