build(deps): bump actions/checkout from 2 to 3

Bumps [actions/checkout](https://github.com/actions/checkout) from 2 to 3.
- [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/v2...v3)

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

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

.github/workflows/docs.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     environment: docs
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       # NOTE: Comment out when https://github.com/rust-lang/mdBook/pull/1306 is merged and released
       # - name: Setup mdBook