docs: reorganize ToC into thematic groups and update contributing guide

Group the Table of Contents entries under bold section headers (AI & ML,
Web & API, Data & Science, etc.) so the README is easier to navigate at
a glance. Update CONTRIBUTING.md to reflect that new sections should be
placed under the appropriate thematic group instead of in a flat list.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: vinta

CONTRIBUTING.md

@@ -80,8 +80,9 @@ Use the **PyPI package name** as the display name so developers can copy it dire
 ## Adding a New Section
 
 1. Add section description in italics: `*Libraries for doing X.*`
-2. Add the section title to the Table of Contents
-3. Keep sections in alphabetical order
+2. Add the section under the appropriate thematic group (e.g., **AI & ML**, **Web & API**)
+3. Add the section title to the Table of Contents under its group
+4. Keep entries in alphabetical order within each category
 
 ## Review Process