Bump actions/checkout from 3 to 5 (#18)

Bumps [actions/checkout](https://github.com/actions/checkout) from 3 to 5.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v3...v5)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '5'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

.github/workflows/pages.yaml

@@ -12,7 +12,7 @@ jobs:
       PYTHON: ""
     steps:
       - name: Checkout this repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v5
 
       - name: Install Julia
         uses: julia-actions/setup-julia@v2

.github/workflows/validate-notebook-index.yml

@@ -21,7 +21,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v5
 
       - name: Set up Julia
         uses: julia-actions/setup-julia@v1