[alerting_example] Migrate deprecated EuiPage* components (#168299)

## Summary

> Note: I strongly recommending [viewing the diff with whitespace
changes turned
off](https://github.com/elastic/kibana/pull/168300/files?w=1), as many
of the "changes" are simply indentation changes.

EUI will shortly be removing several deprecated `EuiPage*` components,
and we're updating a few remaining Kibana usages of these deprecated
components ahead of time.

⚠️ PLEASE NOTE: We have **not** QA'd these changes to ensure that the UI
is 1:1 from before. We do not have the domain knowledge or time to spin
up local instances of all plugins with deprecations, and ask that the
CODEOWNING team pull down this branch and QA the affected page(s)
locally to ensure the UI looks like the same as production. ⚠️

See #161872 for other similar
tasks with more information about this effort.

x-pack/examples/alerting_example/public/components/documentation.tsx

@@ -10,10 +10,7 @@ import React from 'react';
 import {
   EuiText,
   EuiPageBody,
-  EuiPageContent_Deprecated as EuiPageContent,
-  EuiPageContentBody_Deprecated as EuiPageContentBody,
-  EuiPageContentHeader_Deprecated as EuiPageContentHeader,
-  EuiPageContentHeaderSection_Deprecated as EuiPageContentHeaderSection,
+  EuiPageSection,
   EuiPageHeader,
   EuiPageHeaderSection,
   EuiTitle,
@@ -34,33 +31,29 @@ export const DocumentationPage = (
         </EuiTitle>
       </EuiPageHeaderSection>
     </EuiPageHeader>
-    <EuiPageContent>
-      <EuiPageContentHeader>
-        <EuiPageContentHeaderSection>
-          <EuiTitle>
-            <h2>Documentation links</h2>
-          </EuiTitle>
-        </EuiPageContentHeaderSection>
-      </EuiPageContentHeader>
-      <EuiPageContentBody>
-        <EuiText>
-          <h2>Plugin Structure</h2>
-          <p>
-            This example solution has both `server` and a `public` plugins. The `server` handles
-            registration of example the RuleTypes, while the `public` handles creation of, and
-            navigation for, these rule types.
-          </p>
-          <EuiCallOut title="Transport Layer Security" iconType="warning" color="warning">
-            If you see a message about needing to enable the Transport Layer Security, start ES with{' '}
-            <code>yarn es snapshot --ssl --license trial</code> and Kibana with{' '}
-            <code>yarn start --run-examples --ssl</code>. If you running chrome on a mac, you may
-            need to type in <code>thisisunsafe</code> if you see the Certificate invalid screen with
-            no way to &lsquo;proceed anyway&rsquo;.
-          </EuiCallOut>
-        </EuiText>
-        <EuiSpacer />
-        <CreateAlert {...deps} />
-      </EuiPageContentBody>
-    </EuiPageContent>
+    <EuiPageSection>
+      <EuiPageHeader>
+        <EuiTitle>
+          <h2>Documentation links</h2>
+        </EuiTitle>
+      </EuiPageHeader>
+      <EuiText>
+        <h2>Plugin Structure</h2>
+        <p>
+          This example solution has both `server` and a `public` plugins. The `server` handles
+          registration of example the RuleTypes, while the `public` handles creation of, and
+          navigation for, these rule types.
+        </p>
+        <EuiCallOut title="Transport Layer Security" iconType="warning" color="warning">
+          If you see a message about needing to enable the Transport Layer Security, start ES with{' '}
+          <code>yarn es snapshot --ssl --license trial</code> and Kibana with{' '}
+          <code>yarn start --run-examples --ssl</code>. If you running chrome on a mac, you may need
+          to type in <code>thisisunsafe</code> if you see the Certificate invalid screen with no way
+          to &lsquo;proceed anyway&rsquo;.
+        </EuiCallOut>
+      </EuiText>
+      <EuiSpacer />
+      <CreateAlert {...deps} />
+    </EuiPageSection>
   </EuiPageBody>
 );