[Github] Test docs-flang-man target in documentation job (#142860)

This was recently added in 47171ac. We
should be supporting testing this inside the docs job.

Author: boomanaiden154

.github/workflows/docs.yml

@@ -205,7 +205,7 @@ jobs:
           steps.docs-changed-subprojects.outputs.workflow_any_changed == 'true'
         run: |
           cmake -B flang-build -GNinja -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS="clang;mlir;flang" -DLLVM_ENABLE_SPHINX=ON ./llvm
-          TZ=UTC ninja -C flang-build docs-flang-html
+          TZ=UTC ninja -C flang-build docs-flang-html docs-flang-man
           mkdir built-docs/flang
           cp -r flang-build/docs/* built-docs/flang/
       - name: Upload docs