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](actions/checkout@v2...f43a0e5)

---
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:
     env:
       DATADEPS_ALWAYS_ACCEPT: true # bypass confirmation before downloading data (needed for CI)
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@f43a0e5ff2bd294095638e18286ca9a3d1956744
       - uses: julia-actions/setup-julia@latest
         with:
           version: '1'