docs: add conventions and scene configuration guides

[juelg]

Summary

• add a dedicated RCS conventions guide covering quaternion order, frame conventions, Euler angles, gripper semantics, units, and MuJoCo depth/quaternion caveats
• add a sim scene configuration guide explaining SimEnvCreatorConfig, key nested config objects, frame relationships, and example setup patterns from the current scene/config API
• improve discoverability by linking the new pages from the docs landing page, user guide index, and main README
• add small ASCII diagrams for the axis convention and scene frame hierarchy

Details

This PR is based on the latest master and documents the current scene/config API from:

• python/rcs/envs/scenes.py
• python/rcs/envs/configs.py

The new docs also explicitly call out:

• RCS quaternion order is xyzw
• MuJoCo free-joint state uses wxyz
• robot/base frame is x forward, y left, z up
• gripper convention is 0 = closed, 1 = open
• Euler angles are roll, pitch, yaw around x, y, z
• translations are in meters, angles in radians
• depth frames are stored as metric depth scaled by BaseCameraSet.DEPTH_SCALE = 1000 in uint16

Validation

• sphinx-build -b html docs docs/_build/html

Closes #270.