Bump actions/checkout from 5 to 6 (#58)

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

Author: dependabot[bot]

.github/workflows/mdbook.yml

@@ -33,7 +33,7 @@ jobs:
       MDBOOK_VERSION: 0.4.45
       MDBOOK_ADMONISH_VERSION: 1.19.0 # need to re-run mdbook-admonish install if updating version
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Install mdBook
         run: ./.github/install-mdbook
       - name: Setup Pages