docs: update R tutorial to modern Rvolesti syntax #343

[saksham-stack]

Description

This PR updates the "Essential volesti tutorial in R" which was outdated.
Specifically, it updates function names to the modern snake_case API used in Rvolesti.

Changes:

• Updated GenCube to gen_cube.
• Updated sample_points parameters to match current documentation.
• Fixed Hpolytope$new to the modern make_h_polytope constructor.
• Corrected the file path logic for the Birkhoff polytope example using system.file.
• Updated rounding functions (GenSkinnyCube -> gen_skinny_cube).

Fixes #343