updated toc

_toc.yml

@@ -1,39 +1,53 @@
 # PBDL Table of content (cf https://jupyterbook.org/customize/toc.html)
 #
+
 - file: intro.md
-  sections:
+
+- part: Introduction
+  chapters:
   - file: intro-teaser.ipynb
-- file: overview.md
+  - file: overview.md
     sections:
       - file: overview-equations.md
       - file: overview-burgers-forw.ipynb
       - file: overview-ns-forw.ipynb
-- file: supervised.md
+  - file: supervised.md
     sections:
       - file: supervised-airfoils.ipynb
       - file: supervised-discuss.md
-- file: physicalloss.md
+
-  sections:
+- part: Physical Losses
+  chapters:
+  - file: physicalloss.md
   - file: physicalloss-code.ipynb
   - file: physicalloss-discuss.md
-- file: diffphys.md
+
+- part: Differentiable Physics
+  chapters:
+  - file: diffphys.md
     sections:
       - file: diffphys-code-burgers.ipynb
       - file: diffphys-discuss.md
       - file: diffphys-code-ns.ipynb
       - file: diffphys-dpvspinn.md
-- file: diffphys-examples.md
+  - file: diffphys-examples.md
     sections:
       - file: diffphys-code-sol.ipynb
       - file: diffphys-control.ipynb
       - file: diffphys-outlook.md
-- file: physgrad.md
+
+- part: Physical Gradients
+  chapters:
+  - file: physgrad.md
     sections:
       - file: physgrad-comparison.ipynb
       - file: physgrad-nn.md
       - file: physgrad-discuss.md
-- file: outlook.md
+  
-- file: old-phiflow1.md
+- part: End Matter
+  chapters:
+  - file: outlook.md
+  - file: old-phiflow1.md
     sections:
       - file: overview-burgers-forw-v1.ipynb
       - file: overview-ns-forw-v1.ipynb
@@ -42,5 +56,8 @@
       - file: diffphys-code-tf.ipynb
       - file: diffphys-code-ns-v1.ipynb
       - file: diffphys-code-ns-v2a.ipynb
-- file: references.md
+  - file: references.md
-- file: notation.md
+  - file: notation.md
+  
+  
+