(build) added .net 5.0 sdk to docs workflow

Author: arturcic

.github/workflows/docs.yml

@@ -131,6 +131,11 @@ jobs:
       uses: actions/setup-dotnet@v1
       with:
         dotnet-version: '2.1.x'
+    -
+      name: Install .NET SDK 5.0.x
+      uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: '5.0.x'
     -
       name: Install .NET SDK 6.0.x
       uses: actions/setup-dotnet@v1