Bump peaceiris/actions-mdbook from 1 to 2

Bumps [peaceiris/actions-mdbook](https://github.com/peaceiris/actions-mdbook) from 1 to 2.
- [Release notes](https://github.com/peaceiris/actions-mdbook/releases)
- [Changelog](https://github.com/peaceiris/actions-mdbook/blob/main/CHANGELOG.md)
- [Commits](peaceiris/actions-mdbook@v1...v2)

---
updated-dependencies:
- dependency-name: peaceiris/actions-mdbook
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

.github/workflows/docs.yaml

@@ -31,7 +31,7 @@ jobs:
         with:
           fetch-depth: 0
       - name: Setup mdBook
-        uses: peaceiris/actions-mdbook@v1
+        uses: peaceiris/actions-mdbook@v2
         with:
           mdbook-version: 'latest'
       - name: Install mdbook plugins