ci_docs: build fusion docs (#15127)

.github/workflows/ci_docs.yml

@@ -11,6 +11,7 @@ on:
       - 'tools/dochack/dochack.nim'
       - 'tools/kochdocs.nim'
       - '.github/workflows/ci_docs.yml'
+      - 'koch.nim'
   pull_request:
     # Run only on changes on these files.
     paths:
@@ -121,6 +122,10 @@ jobs:
         shell: bash
         run: ./koch boot -d:release
 
+      - name: 'Clone fusion'
+        shell: bash
+        run: ./koch fusion
+
       - name: 'Build documentation'
         shell: bash
         run: ./koch doc --git.commit:devel