docs: fix gh-pages asset 404s by passing --root to fsdocs build

Without --root, fsdocs generates asset hrefs like /fsdocs-theme.js which
resolve to the bare domain root. Passing the GitHub Pages base URL fixes
them to /FSharp.Control.TaskSeq/fsdocs-theme.js.

Author: dsyme

.github/workflows/docs.yaml

@@ -35,7 +35,7 @@ jobs:
         run: dotnet build src/FSharp.Control.TaskSeq/FSharp.Control.TaskSeq.fsproj -c Release
 
       - name: Build docs
-        run: dotnet fsdocs build --properties Configuration=Release --eval
+        run: dotnet fsdocs build --properties Configuration=Release --eval --root https://fsprojects.github.io/FSharp.Control.TaskSeq/
 
       - name: Deploy to GitHub Pages
         uses: peaceiris/actions-gh-pages@v4