build(deps-dev): Bump watchpack from 2.3.1 to 2.4.0 in /docs (#8700)

Bumps [watchpack](https://github.com/webpack/watchpack) from 2.3.1 to 2.4.0.
- [Release notes](https://github.com/webpack/watchpack/releases)
- [Commits](https://github.com/webpack/watchpack/compare/v2.3.1...v2.4.0)

---
updated-dependencies:
- dependency-name: watchpack
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

docs/package.json

@@ -8,7 +8,7 @@
   },
   "devDependencies": {
     "@vuepress/plugin-html-redirect": "^0.1.4",
-    "watchpack": "^2.3.1"
+    "watchpack": "^2.4.0"
   },
   "scripts": {
     "preserve": "./pre.sh",