[docs] Option to disable ads

[oliviertassinari]

Ads are meant to sustain the MIT licensed code. I don't think that it makes sense to keep them for code released under a paid commercial license. This change allows https://github.com/mui-org/material-ui-x to disable ads, where relevant.

Usage example:

diff --git a/docs/pages/components/alert.js b/docs/pages/components/alert.js
index caeeb16f23..54f9f2bec9 100644
--- a/docs/pages/components/alert.js
+++ b/docs/pages/components/alert.js
@@ -11,7 +11,7 @@ const requireRaw = require.context(
 );

 export default function Page({ demos, docs }) {
-  return <MarkdownDocs demos={demos} docs={docs} requireDemo={requireDemo} />;
+  return <MarkdownDocs demos={demos} docs={docs} disableAd requireDemo={requireDemo} />;
 }

 Page.getInitialProps = () => {
diff --git a/docs/src/pages/components/alert/alert.md b/docs/src/pages/components/alert/alert.md
index f77f0250f8..5209e349ae 100644
--- a/docs/src/pages/components/alert/alert.md
+++ b/docs/src/pages/components/alert/alert.md
@@ -18,7 +18,7 @@ waiAria: https://www.w3.org/TR/wai-aria-practices/#alert

 The alert offers four severity levels that set a distinctive icon and color.

-{{"demo": "pages/components/alert/SimpleAlerts.js"}}
+{{"demo": "pages/components/alert/SimpleAlerts.js", "disableAd": true}}

 ## Description

Hopefully, one day, we will be able to kill all the ads.

[mui-pr-bot]

No bundle size changes comparing e4d6eac...2996d22

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