ci(docs): use unsafe-best-match index strategy for docs install

The new UV_EXCLUDE_NEWER guard combined with the PyTorch CPU extra-index
caused uv to pin fsspec to an older mirrored version that conflicts with
lightning[all]'s version range, breaking docs-build resolution.

Author: bhimrazy

.github/workflows/docs-build.yml

@@ -105,9 +105,15 @@ jobs:
 
       - name: Install package & dependencies
         timeout-minutes: 20
+        # `unsafe-best-match` lets uv consider all indexes when resolving a package,
+        # not just the first one it's found on. Required here because the PyTorch CPU
+        # extra-index mirrors an older `fsspec` that conflicts with `lightning[all]`'s
+        # version range under `UV_EXCLUDE_NEWER`; without this flag uv refuses to fall
+        # back to PyPI for a newer compatible version.
         run: |
           uv pip install .[all] -U -r requirements/${{ matrix.pkg-name }}/docs.txt \
-            -f ${PYPI_LOCAL_DIR} --extra-index-url="${TORCH_URL}"
+            -f ${PYPI_LOCAL_DIR} --extra-index-url="${TORCH_URL}" \
+            --index-strategy unsafe-best-match
           uv pip list
 
       - name: Install req. for Notebooks/tutorials