[Proposal] Add URDF Assembly Tool Documentation

[yuecideng]

Overview

Add comprehensive documentation for the URDF (Unified Robot Description Format) assembly tool within the repository. This should include usage instructions, example configurations, and explanations of the tool's key features.

Task Details

• Create documentation files at docs/sources/features directory for this feature.
• Include usage examples, command-line options (if any).
• Explain the main features, expected inputs, and typical outputs of the tool.
• Link to related resources or specifications if applicable

Motivation

Proper documentation will help users and contributors understand, use, and extend the URDF assembly tool effectively, improving project adoption and onboarding.

Acceptance Criteria

• Documentation is clear, concise, and covers all main aspects of the URDF assembly tool
• Usage examples and sample files are included
• All relevant files are updated or created as needed

[yuecideng]

Close with PR: #174