chore: Don't build the docs for xtask

Building the docs for xtask spews a bunch of unexpected cfg warnings. As
these warnings come from a macro in a dependency and the docs for xtask
don't exist nor will, let's just not build them with the rest of the
docs.

.github/workflows/documentation.yml

@@ -53,7 +53,7 @@ jobs:
         env:
           RUSTDOCFLAGS: "--enable-index-page -Zunstable-options --cfg docsrs -Dwarnings"
         run:
-          cargo doc --no-deps --workspace --features docsrs
+          cargo doc --no-deps --workspace --features docsrs --exclude=xtask
 
       - name: Upload artifact
         if: github.event_name == 'push' && github.ref == 'refs/heads/main'