[9.0] ESQL: Add reference entry for LOOKUP JOIN (#210201) (#210268)

# Backport

This will backport the following commits from `main` to `9.0`:
- [ESQL: Add reference entry for LOOKUP JOIN
(#210201)](#210201)

<!--- Backport version: 9.4.3 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sqren/backport)

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this builds correctly, as I couldn't quickly whip\r\nup a dev
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Add reference entry for LOOKUP JOIN (#210201)\n\nI couldn't verify if
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Co-authored-by: Alexander Spies <[email protected]>

src/platform/packages/private/kbn-language-documentation/src/sections/esql_documentation_sections.tsx

@@ -397,10 +397,61 @@ FROM employees
         />
       ),
     },
+    {
+      label: i18n.translate('languageDocumentation.documentationESQL.lookupJoin', {
+        defaultMessage: 'LOOKUP JOIN',
+      }),
+      preview: true,
+      description: (
+        <Markdown
+          openLinksInNewTab={true}
+          markdownContent={i18n.translate(
+            'languageDocumentation.documentationESQL.lookupJoin.markdown',
+            {
+              defaultMessage: `### LOOKUP JOIN
+You can use \`LOOKUP JOIN\` to add data from an existing index to incoming rows. While this is similar to \`ENRICH\`, it does not require an enrich policy to be executed beforehand. Additionally, if multiple matching documents are found in the lookup index, they will generate multiple output rows.
+
+\`\`\`
+ROW language_code = 1
+| LOOKUP JOIN languages ON language_code
+\`\`\`
+
+An index that is used in \`LOOKUP JOIN\` needs to be in lookup mode. To create a lookup index, set the index mode to lookup.
+
+\`\`\`
+PUT languages
+'{
+  "settings": {
+    "index":{
+      "mode":"lookup"
+    }
+  }
+}'
+\`\`\`
+
+The join key field must have a compatible type and match the name of the field in the lookup index to find matching documents. You can use \`RENAME\` or \`EVAL\` to rename columns as needed.
+
+\`\`\`
+FROM employees
+| EVAL language_code = languages
+| LOOKUP JOIN languages ON language_code
+\`\`\`
+
+In case of name collisions, the fields from the lookup index will override the existing fields.
+            `,
+              ignoreTag: true,
+              description:
+                'Text is in markdown. Do not translate function names, special characters, or field names like sum(bytes)',
+            }
+          )}
+        />
+      ),
+    },
     {
       label: i18n.translate('languageDocumentation.documentationESQL.mvExpand', {
         defaultMessage: 'MV_EXPAND',
       }),
+      preview: true,
       description: (
         <Markdown
           markdownContent={i18n.translate(