docs(v2): config for self-hosted docsearch crawler (#4524)

website/docs/search.md

@@ -33,6 +33,9 @@ module.exports = {
       // Optional: see doc section below
       contextualSearch: true,
 
+      // Optional: see doc section below
+      appId: 'YOUR_APP_ID',
+
       // Optional: Algolia search parameters
       searchParameters: {},
 
@@ -80,6 +83,24 @@ When using `contextualSearch: true`, the contextual facet filters will be merged
 
 :::
 
+### Custom Application ID {#custom-application-id}
+
+When [running your own](https://docsearch.algolia.com/docs/run-your-own/) DocSearch crawler, it is [required to set the `appId` configuration key](https://docsearch.algolia.com/docs/behavior/#appid) to your own Application ID. If left unset, the `appId` will fallback to the one used with the free, hosted version of Algolia DocSearch.
+
+```jsx title="docusaurus.config.js"
+module.exports = {
+  // ...
+  themeConfig: {
+    // ...
+    // highlight-start
+    algolia: {
+      appId: 'YOUR_APP_ID',
+    },
+    // highlight-end
+  },
+};
+```
+
 ### Styling your Algolia search {#styling-your-algolia-search}
 
 By default, DocSearch comes with a fine-tuned theme that was designed for accessibility, making sure that colors and contrasts respect standards.