[Github] Add libunwind to docs CI (#69830)

This patch adds the libunwind docs to the Github docs action which
enables easy triage of docs build failures in Github PRs. There is
already buildbot coverage of this configuration, but it is much less
convenient to use in PRs.

.github/workflows/docs.yml

@@ -17,12 +17,14 @@ on:
       - 'clang/docs/**'
       - 'clang-tools-extra/docs/**'
       - 'lldb/docs/**'
+      - 'libunwind/docs/**'
   pull_request:
     paths:
       - 'llvm/docs/**'
       - 'clang/docs/**'
       - 'clang-tools-extra/docs/**'
       - 'lldb/docs/**'
+      - 'libunwind/docs/**'
 
 jobs:
   check-docs-build:
@@ -58,6 +60,8 @@ jobs:
               - 'clang-tools-extra/docs/**'
             lldb:
               - 'lldb/docs/**'
+            libunwind:
+              - 'libunwind/docs/**'
       - name: Setup Python env
         uses: actions/setup-python@v4
         with:
@@ -91,4 +95,9 @@ jobs:
         run: |
           cmake -B lldb-build -GNinja -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS="clang;lldb" -DLLVM_ENABLE_SPHINX=ON ./llvm
           TZ=UTC ninja -C lldb-build docs-lldb-html docs-lldb-man
+      - name: Build libunwind docs
+        if: steps.docs-changed-subprojects.outputs.libunwind_any_changed == 'true'
+        run: |
+          cmake -B libunwind-build -GNinja -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_RUNTIMES="libunwind" -DLLVM_ENABLE_SPHINX=ON ./runtimes
+          TZ=UTC ninja -C libunwind-build docs-libunwind-html