Bump nunit/docfx-action from 3.4.2 to 4.0.0 (#1614)

Bumps [nunit/docfx-action](https://github.com/nunit/docfx-action) from 3.4.2 to 4.0.0.
- [Release notes](https://github.com/nunit/docfx-action/releases)
- [Commits](nunit/docfx-action@v3.4.2...v4.0.0)

---
updated-dependencies:
- dependency-name: nunit/docfx-action
  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>

Author: dependabot[bot]

.github/workflows/docfx.yaml

@@ -39,7 +39,7 @@ jobs:
     - name: Build
       run: dotnet build -c Release
 
-    - uses: nunit/docfx-action@v3.4.2
+    - uses: nunit/docfx-action@v4.0.0
       name: Build Documentation
       with:
         args: doc/docfx.json