[Github] Add lld to docs CI (#69821)

This patch adds the lld documentation to the documentation github
actions CI to automatically validate in PRs/at tip of tree that the docs
build and there aren't any Sphinx warnings. There is existing buildbot
coverage for the lld docs, but this much more convienient to use in
cases like PRs.

.github/workflows/docs.yml

@@ -20,6 +20,7 @@ on:
       - 'libunwind/docs/**'
       - 'libcxx/docs/**'
       - 'libc/docs/**'
+      - 'lld/docs/**'
   pull_request:
     paths:
       - 'llvm/docs/**'
@@ -29,6 +30,7 @@ on:
       - 'libunwind/docs/**'
       - 'libcxx/docs/**'
       - 'libc/docs/**'
+      - 'lld/docs/**'
 
 jobs:
   check-docs-build:
@@ -63,6 +65,8 @@ jobs:
               - 'libcxx/docs/**'
             libc:
               - 'libc/docs/**'
+            lld:
+              - 'lld/docs/**'
       - name: Fetch LLVM sources (PR)
         if: ${{ github.event_name == 'pull_request' }}
         uses: actions/checkout@v4
@@ -116,4 +120,9 @@ jobs:
         run: |
           cmake -B libc-build -GNinja -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_RUNTIMES="libc" -DLLVM_ENABLE_SPHINX=ON ./runtimes
           TZ=UTC ninja -C docs-libc-html
+      - name: Build LLD docs
+        if: steps.docs-changed-subprojects.outputs.lld_any_changed == 'true'
+        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