Add shared RS/DM D405, D435i and Gemini 2 wrist camera descriptions

[Yang-Ci]

Adds wrist camera descriptions shared by B601-RS and B601-DM for RealSense D405, RealSense D435i and Orbbec Gemini 2 under Rebot_Arm_description/Camera/. Each generated URDF includes the vendor camera body, bracket, mounting screw offsets and optical frames, rooted at an empty gripper_end attachment reference.

Both arms reuse the camera bodies, brackets, meshes and assembly URDFs. RS attaches to its existing gripper_end link; DM uses end_link and requires a fixed mounting transform to align the reference frames. The bilingual camera README and both description-package READMEs explain this shared use. The three existing MuJoCo screenshots illustrate the shared assemblies on RS.

The directory includes all referenced STL/Collada meshes, bundled upstream Xacros, a regeneration script, source revisions and SHA-256 hashes, and Apache-2.0/CERN-OHL-W-2.0 license texts. Its bilingual Chinese/English README shows the three tested MuJoCo assembly screenshots and explains attachment, mounting parameters and ROS resource paths. Both existing description-package READMEs link to the new resources.

Validation:

• Parsed all three URDFs, checked their fixed-joint trees and confirmed every mesh reference resolves.
• Confirmed the RS arm provides the gripper_end attachment link.
• Regenerated all three URDFs from the bundled Xacros with byte-identical output.
• Verified all 23 original source-asset hashes, including staged Git blobs.
• Confirmed all camera README local links resolve and the three screenshots retain their original bytes.
• git diff --cached --check passed.

CI note: the previous repository-wide link-check run reports six pre-existing email links in the root README_zh.md and README_JP.md as missing local files. All six are already present in the base branch; neither file is changed by this PR. No errors were reported for the new camera documentation. Check log.