docs(index[toctree]): add gp-furo-theme + gp-sphinx-vite to Internal toctree

why: docs CI builds with `sphinx-build -W` (warnings as errors). Both
new package pages emitted toc.not_included warnings since the previous
diff added their .md files but never wired them into a toctree —
{workspace-package-grid} renders link cards but doesn't satisfy
Sphinx's toctree-membership check.

what:
- Append packages/gp-furo-theme + packages/gp-sphinx-vite under the
  existing "Internal" caption, alongside gp-sphinx + sphinx-gp-theme.

Author: tony

docs/index.md

@@ -132,6 +132,8 @@ packages/sphinx-autodoc-typehints-gp
 
 packages/gp-sphinx
 packages/sphinx-gp-theme
+packages/gp-furo-theme
+packages/gp-sphinx-vite
 ```
 
 ```{toctree}