Add documentation to add custom admonitions

Hello,
I found myself needing to add a custom admonition, and thought it would be worth adding documentation on how to do so. 

I've considered adding the admonition itself to this repository to be able to preview it in the docs, but I didn't want to propose a config change unless you agree beforehand.

website/docs/guides/markdown-features/markdown-features-admonitions.mdx

@@ -121,3 +121,35 @@ import TabItem from '@theme/TabItem';
 ```
 
 :::
+
+## Custom Admonitions
+
+If you're using `@docusaurus/preset-classic`, you can create custom admonitions by adding them to your website's `docusaurus.config.js`. 
+
+For example, the following configuration adds an admonition with the `custom` keyword. This admonition will be styled as an [Infima `alert-info` component](https://infima.dev/docs/components/alert):
+
+```js title="docusaurus.config.js"
+module.exports = {
+  // ...
+  presets: [
+    [
+      "@docusaurus/preset-classic",
+      {
+        docs: {
+          // ...
+          admonitions: {
+            customTypes: {
+              custom: {
+                ifmClass: "info",
+                svg: '<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16"><path fill-rule="evenodd" d="M15 2H1c-.55 0-1 .45-1 1v9c0 .55.45 1 1 1h5.34c-.25.61-.86 1.39-2.34 2h8c-1.48-.61-2.09-1.39-2.34-2H15c.55 0 1-.45 1-1V3c0-.55-.45-1-1-1zm0 9H1V3h14v8z"></path></svg>'
+              },
+            },
+          },
+        },
+      }
+    ]
+  ]
+};
+```
+
+Further documentation is available on [Remark's own repository](https://github.com/elviswolcott/remark-admonitions#custom-types).