Update TOC structure

Author: benjaoming

docs/_toc.yml

@@ -3,12 +3,16 @@
 
 format: jb-book
 root: intro
-chapters:
-- file: markdown
-- file: notebooks
-- file: markdown-notebooks
-- file: intersphinx
-- file: sphinx-examples
-- file: sphinx-hoverxref
-- file: sphinx-proof
+parts:
+- caption: Basic examples 🪄
+  chapters:
+  - file: markdown
+  - file: notebooks
+  - file: markdown-notebooks
+- caption: Cool extensions 🕶️
+  chapters:
+  - file: intersphinx
+  - file: sphinx-examples
+  - file: sphinx-hoverxref
+  - file: sphinx-proof