build(deps-dev): bump postcss from 8.5.25 to 8.5.26 in /docs

Bumps [postcss](https://github.com/postcss/postcss) from 8.5.25 to 8.5.26.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.5.25...8.5.26)

---
updated-dependencies:
- dependency-name: postcss
  dependency-version: 8.5.26
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

docs/package-lock.json

@@ -18,7 +18,7 @@
   "homepage": "https://github.com/pipe-cd/pipecd#readme",
   "devDependencies": {
     "autoprefixer": "^10.5.4",
-    "postcss": "^8.5.25",
+    "postcss": "^8.5.26",
     "postcss-cli": "^11.0.1"
   }
 }