Bump peaceiris/actions-mdbook from 1.2.0 to 2.0.0 (ZcashFoundation#637)

Bumps [peaceiris/actions-mdbook](https://github.com/peaceiris/actions-mdbook) from 1.2.0 to 2.0.0.
- [Release notes](https://github.com/peaceiris/actions-mdbook/releases)
- [Changelog](https://github.com/peaceiris/actions-mdbook/blob/main/CHANGELOG.md)
- [Commits](peaceiris/actions-mdbook@v1.2.0...v2.0.0)

---
updated-dependencies:
- dependency-name: peaceiris/actions-mdbook
  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>

.github/workflows/docs.yml

@@ -50,7 +50,7 @@ jobs:
       - uses: Swatinem/rust-cache@v2
 
       - name: Setup mdBook
-        uses: peaceiris/actions-mdbook@v1.2.0
+        uses: peaceiris/actions-mdbook@v2.0.0
         with:
           mdbook-version: '0.4.18'