[Github] Add OpenMP docs to Github docs action (#70529)

This patch enables building the openmp documentation through Github
actions during PRs and at tip of tree to ensure that the documentation
builds and that there aren't any Sphinx warnings in a manner that
enables more rapid developer iteration without having to install
documentation tooling.

Author: boomanaiden154

.github/workflows/docs.yml

@@ -21,6 +21,7 @@ on:
       - 'libcxx/docs/**'
       - 'libc/docs/**'
       - 'lld/docs/**'
+      - 'openmp/docs/**'
   pull_request:
     paths:
       - 'llvm/docs/**'
@@ -31,6 +32,7 @@ on:
       - 'libcxx/docs/**'
       - 'libc/docs/**'
       - 'lld/docs/**'
+      - 'openmp/docs/**'
 
 jobs:
   check-docs-build:
@@ -67,6 +69,8 @@ jobs:
               - 'libc/docs/**'
             lld:
               - 'lld/docs/**'
+            openmp:
+              - 'openmp/docs/**'
       - name: Fetch LLVM sources (PR)
         if: ${{ github.event_name == 'pull_request' }}
         uses: actions/checkout@v4
@@ -125,4 +129,9 @@ jobs:
         run: |
           cmake -B lld-build -GNinja -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS="lld" -DLLVM_ENABLE_SPHINX=ON ./llvm
           TZ=UTC ninja -C lld-build docs-lld-html
+      - name: Build OpenMP docs
+        if: steps.docs-changed-subprojects.outputs.openmp_any_changed == 'true'
+        run: |
+          cmake -B openmp-build -GNinja -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS="clang;openmp" -DLLVM_ENABLE_SPHINX=ON ./llvm
+          TZ=UTC ninja -C openmp-build docs-openmp-html