Local hosting option for AI Assistant KB artifacts missing index.xml requirement

## Page

https://www.elastic.co/docs/explore-analyze/ai-features/ai-chat-experiences/ai-assistant-host-doc-artifacts

## Problem

The **Local files on the Kibana host** section instructs users to place the ZIP files in a directory and set `xpack.productDocBase.artifactRepositoryUrl` to a `file://` URL pointing at that directory. It does not mention any additional files being required.

However, the Kibana source code shows that when a `file://` URL is used, Kibana does **not** scan the directory for ZIPs. Instead, it unconditionally looks for an `index.xml` file in that directory and parses it as an S3-style `ListBucketResult` XML listing. If `index.xml` is absent, artifact discovery fails with no useful error surfaced to the user.

Relevant code in [`fetch_artifact_versions.ts`](https://github.com/elastic/kibana/blob/v8.19.15/x-pack/platform/plugins/shared/ai_infra/product_doc_base/server/services/package_installer/steps/fetch_artifact_versions.ts) (identical in 8.19.15 and 9.1.x):

```typescript
function fetchLocalFile(parsedUrl: URL): Promise {
return new Promise((resolve, reject) => {
const normalizedPath = resolveLocalArtifactsPath(parsedUrl);
const xmlFilePath = Path.join(normalizedPath, 'index.xml'); // hardcoded

fs.readFile(xmlFilePath, (err, data) => {
if (err) {
reject(err); // no index.xml = immediate rejection, no fallback
} else {
resolve(data);
}
});
});
}
```

And the caller:

```typescript
if (parsedUrl.protocol === 'file:') {
const file = await fetchLocalFile(parsedUrl); // always requires index.xml
xml = file.toString();
} else {
const res = await fetch(`${artifactRepositoryUrl}?max-keys=1000`);
xml = await res.text();
}
```

## What the docs should add

The local files section should state that `index.xml` must be present in the same directory as the ZIPs, using the same `ListBucketResult` XML format described in the CDN section. For example, for Kibana 8.19:

```xml

kibana-ai-assistant-kb-artifacts
false

kb-product-doc-elasticsearch-8.19.zip


kb-product-doc-kibana-8.19.zip


kb-product-doc-observability-8.19.zip


kb-product-doc-security-8.19.zip

```

The expected directory layout should also be shown:

```
/path/to/artifacts/
├── index.xml
├── kb-product-doc-elasticsearch-..zip
├── kb-product-doc-kibana-..zip
├── kb-product-doc-observability-..zip
└── kb-product-doc-security-..zip
```

## Impact

Users following the current docs will silently fail to load KB artifacts with no clear error.