Update GitHub Actions used in docs.yml

- Updates to latest versions for all actions
- Pin actions not maintained by GitHub under `github.com/action` to exact
  commit hashes. This is good practice for security, since tags can be
  moved and then arbitray new code may execute. Since updating commit
  hashes is slightly more hassle, the trade-off made here is to trust
  GitHub, but not third-party actions.

This should also solve the warnings about Node.js 16 actions being
deprecated that is currently visible on the log page when docs.yml
is executed.

.github/workflows/docs.yml

@@ -9,15 +9,15 @@ jobs:
     if: "github.repository == 'OpenMathLib/OpenBLAS'"
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
-      - uses: actions/setup-python@v2
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
         with:
           python-version: "3.10"
       - run: pip install mkdocs mkdocs-material
       # mkdocs gh-deploy command only builds to the top-level, hence building then deploying ourselves
       - run: mkdocs build
       - name: Deploy docs
-        uses: peaceiris/actions-gh-pages@v3
+        uses: peaceiris/actions-gh-pages@4f9cc6602d3f66b9c108549d475ec49e8ef4d45e # v4.0.0
         if: ${{ github.ref == 'refs/heads/develop' }}
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}