ci: 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](https://github.com/actions/checkout/compare/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>

.github/workflows/build_docs.yml

@@ -25,7 +25,7 @@ jobs:
           sudo apt install -y git
 
       - name: Check out code
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           fetch-depth: 0
       - name: Ensure the docs branch
@@ -48,12 +48,12 @@ jobs:
           fi
 
       - name: Checkout the docs branch
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           ref: docs
 
       - name: Fetch README.md and .pandoc_template.html5 template from the workflow branch
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           sparse-checkout: |
             README.md