docs: Pin @rspack/core to 1.5.8 in website folder (#502)

Pins `@rspack/core` to version 1.5.8 in the website folder to prevent
version drift and ensure build consistency.

## Changes

- Added `overrides` field to `website/package.json` specifying
`@rspack/core: "1.5.8"`
- Updated `website/package-lock.json` to reflect the pinned version

The override ensures all transitive dependencies use exactly version
1.5.8, regardless of their declared semver ranges.

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<details>

<summary>Original prompt</summary>

> in wesbite folder, use npm overrides to pin `@rspack/core` to 1.5.8
and update the lockfile


</details>



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website/package-lock.json

@@ -1,4 +1,7 @@
 {
+    "overrides": {
+        "@rspack/core": "1.5.8"
+    },
     "scripts": {
         "examples": "docusaurus-examples",
         "start": "rimraf .docusaurus && CRAWLEE_DOCS_FAST=1 docusaurus start",