Add comprehensive documentation for user account management, notification...

Add comprehensive documentation for user account management, notification management, training workflow, and webhook types

• Created user management documentation covering registration, login, password reset, and change password functionalities.
• Added notification management documentation detailing viewing, marking as read, and deleting notifications.
• Introduced training workflow documentation outlining data preparation, model training, and inference processes.
• Defined webhook types with TypeScript interfaces for device configurations, notifications, and related message types.
• Updated mkdocs configuration for new documentation sections.
• Enhanced frontend header component to include a link to the manual documentation.