Bump actions/checkout from 5 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 5 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v5...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

.github/workflows/documentation.yaml

@@ -12,7 +12,7 @@ jobs:
     runs-on: "ubuntu-latest"
     steps:
       - name: "Checkout"
-        uses: "actions/checkout@v5"
+        uses: "actions/checkout@v6"
 
       - name: "Build"
         uses: "phpDocumentor/phpDocumentor@master"
@@ -32,7 +32,7 @@ jobs:
     needs: "documentation"
     steps:
       - name: "Checkout"
-        uses: "actions/checkout@v5"
+        uses: "actions/checkout@v6"
         with:
           repository: "phpDocumentor/docs"
           token: "${{ secrets.BOT_TOKEN }}"