[Docs] UI - Project Management: Add comprehensive UI documentation with beta notice

Add detailed UI walkthrough for Project Management feature including:
- Beta notice with link to API documentation
- Overview of projects and organizational hierarchy
- Prerequisites and setup instructions
- Separate section for enabling projects in UI settings
- Step-by-step guide for creating and managing projects
- Use cases for key organization within teams
- Next steps and related documentation links
- Proper sidebar navigation integration

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

docs/my-website/sidebars.js

@@ -348,6 +348,7 @@ const sidebars = {
                 "proxy/access_control",
                 "proxy/self_serve",
                 "proxy/public_teams",
+                "proxy/ui_project_management",
                 "proxy/ui/bulk_edit_users",
                 "proxy/ui/page_visibility",
               ]