move Documentation check to separate job

Currently all cabal tests can fail due to missing documentation,
this is undesirable since we can no more distinguish ghc / clash version related problems if documentation does not build

Author: lmbollen

.github/workflows/ci.yml

@@ -111,10 +111,6 @@ jobs:
         run: |
           .ci/test_cabal.sh
 
-      - name: Documentation
-        run: |
-          .ci/build_docs.sh
-
   linting:
     name: Source code linting
     runs-on: ubuntu-latest
@@ -125,3 +121,14 @@ jobs:
       - name: Whitespace
         run: |
           .ci/test_whitespace.sh
+
+  documentation:
+    name: Documentation
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Build
+        run: |
+          .ci/build_docs.sh